Add support for channel tabs and channel tags

[AudricV]

• I carefully read the contribution guidelines and agree to them.
• I have tested the API against NewPipe.
• I agree to create a pull request for NewPipe as soon as possible to make it compatible with the changed API.
  Already done by someone else: Add support for channel tabs NewPipe#9182, which needs to be updated to the latest changes made in this pull request (see below).

This PR supersedes #951 by reusing the changes from @Theta-Dev and @Stypox and applying the latest requested changes where relevant. Thanks a lot for your original work!

It introduces support of channel tabs and tags. The channel tab changes are breaking changes, as ChannelExtractor is now only an Extractor abstract implementation instead of a ListExtractorone: this means that ChannelExtractor do not provide StreamInfoItem anymore. They can be extracted from channel tabs instead.

Channel tabs (which are strings to be compliant with the current LinkHandler filtering and sorting system) need to be fetched separately from channels. A dynamic list of available tabs is returned by ChannelInfo and ChannelExtractor with the method getTabs. The tab list is a list of ListLinkHandlers, which can be used to fetch tabs using the getChannelTabExtractor method.
In some cases, tab data is sent with the channel data, so fetching tabs would require an additional and duplicate request. To avoid this issue, a new custom LinkHandler can be used to create a ChannelTabExtractor instance with the data already fetched in it: ReadyChannelTabListLinkHandler.

Alternatively, a specific tab can be fetched using the corresponding ChannelExtractor methods taking a specific channel tab name (which should be one of the ChannelTabs constants) and returning channel tab extractors. In this case, ReadyChannelTabListLinkHandlers cannot be used, so duplicate requests could happen with this process; passing an invalid or unsupported tab name may return an exception indicating that the given tab name is invalid or unsupported.

Services' channel tab extractors, which so have ChannelTabExtractor as a base abstract class, are ListExtractor implementations allowing any InfoItem type. This results in the ability to provide streams, playlists and channels in channel tabs, and would allow to implement a posts item type in the future to support contents of YouTube's community tab for instance.

Channel tabs could be also fetched with their Info equivalent, ChannelTabInfo.

Note: Contrary to the original PR, channel tabs have their own package, tabs, a package of the channel one, in which ChannelTabExtractor, ChannelTabInfo and ChannelTabs classes are located. The ReadyChannelTabListLinkHandler class, which may need to be renamed, is still in the linkhandler package.

The following channel tabs are supported with this PR:

• for YouTube: Videos (tab's parameters have been updated to use the one provided and used by the desktop website), Shorts (by using only the channel tab except for age-restricted channels, see below), Live, Playlists and Channels;
• for SoundCloud: Tracks, Playlists and Albums;
• for MediaCCC: Videos;
• for PeerTube: Videos, Playlists and Channels;
• for Bandcamp: Tracks and Albums.

What is done with #951 but has been not highlighted is the support of channel tags: a list of strings which can be get using the getTags method of ChannelInfo and ChannelExtractor. Only YouTube seem to provide this information, so support has been added for this service only, other ones return an empty list, the default behavior.

I decided to do a little bit more than copying and improving changes of the original PR in this one: I also added support of detecting age-restricted YouTube channels and getting StreamInfoItems of them using YouTube's system playlists. These playlists may contain geo-restricted (and so unavailable) videos. This only works when getting tabs from a channel, due to the usage of a ReadyChannelTabListLinkHandler and a ChannelTabExtractor implementation wrapping a PlaylistExtractor in this case. This support should be added when getting directly tabs, by moving the corresponding code as a fallback for stream items to YouTube's standard ChannelTabExtractor.

So, for these channels, the Videos, Shorts and Live tabs will be always returned, even if a tab or multiple tabs do or does not exist (it/they would return no items in this case, even if system playlists don't exist when a type of a content has been not created by a channel).
Note that support of the Shorts tab using system playlists is currently broken, as YouTube return a short UI for playlists with only shorts in most cases; this UI is currently not supported by the extractor. See #1077 for more details.

Tests have been of course updated to reflect the changes made in this PR, and the YouTube channels mocks had to be updated.

Closes #227, closes #501 (only support for topic channels which have regular tabs has been added, so system channels such as Music or Gaming return no tabs, this is tracked by #611), closes #585.

Note for reviewers: I recommend to review the changes commit by commit, as they are huge.

Debug APK built by @Stypox: app-debug.zip

[Stypox]

Partial review; I had some time to look at the first few commits (up until the YouTube one, where I stopped at YoutubeChannelExtractor)

[Stypox]

There are a few small things to change, but overall everything is fine. If you don't have objections, I can fix everything myself and then merge. Thank you again to you and Theta-Dev for the effort on this PR!

Btw, for the future try to keep the commit subject under 50 characters: https://stackoverflow.com/q/2290016

[Stypox]

Did we decide on a better name or not in the end?

[Stypox]

Aha! I took back ownership of commits! I rebased and tested locally, everything works fine. I will open a PR for the small suggestions I proposed above, as they are not blocking.