[Bandcamp] Support loading additional comments

[petlyh]

• 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.
  No NewPipe changes required.

The Bandcamp service now supports loading multiple pages of comments.
Closes #621

Initial comments are now extracted from the data-blob attribute of the #collectors-data element, with subsequent pages of comments being fetched from the API. The way initial comments are being extracted was changed to:

1. have the first page include all of the initial comments, not just the ones shown.
2. be able to use the same CommentsInfoItemExtractor for both the initial page and subsequent pages.

Note: I was unable to find a track with more than 2 pages of comments to test.

Before:

After:

after.mp4

[AudricV]

Thank you very much for this PR!

Please apply the following changes and add @Nonnull and @Nullable annotations where that's relevant in the code you added and changed.

You will also have to wait a review from @fynngodau.

The Bandcamp tests failures from the CI doesn't seem to be related to your changes (timeouts).

[fynngodau]

Hello @petlyh, many thanks for opening this PR. I'm happy to see that you have taken a look at this old Bandcamp issue!

Finding a track, not an album, with many comments is indeed quite tricky! I found this one, with three pages (one initial + two): https://laurashigihara.bandcamp.com/track/cube-land

[petlyh]

Finding a track, not an album, with many comments is indeed quite tricky! I found this one, with three pages (one initial + two): https://laurashigihara.bandcamp.com/track/cube-land

Thank you so much! I was now able to verify that loading more pages after the second page works as expected :-)

[Stypox]

Code looks good to me, except for the things already pointed out. I don't know anything about Bandcamp though.

[fynngodau]

(FYI, I'm going to post an approving review once the points from the other reviews are addressed)

[fynngodau]

Thank you for applying the review comments, and many thanks from my side yet again!