Bugfix namespaced filters

[MarkusFreitag]

What this PR does / why we need it:

Currently there is a recursion bug for namespaced filters. It is caused by the loop within the merge closure, which itself is called for each item. This behaviour let to having the hashed namespace been added multiple times.

Which issue(s) this PR fixes:

Fixes the above explained bug.

Does this PR introduced a user-facing change?

None

[benjaminhuo]

cc @wanjunlei

[adiforluls]

I'll take a look tomorrow

[MarkusFreitag]

A bit more context to the individual commits:
The first one fixes the described bug, by replacing the for loop with a switch/case type assertion. I've also added some unittests to verify that.
The interface, introduced within the second one, aims to increase read- and maintainability for the future, as the switch/case would just grow bigger. In case you don't like this solution, it could be dropped. In this case the addition from commit three should still be added as namespaced rewrite-tag filters have not worked before.

[adiforluls]

LGTM, I can approve but there's a pipeline failure.
cc: @benjaminhuo

[benjaminhuo]

LGTM, I can approve but there's a pipeline failure. cc: @benjaminhuo

nevermind the pipeline error, it's trying to push image to ghcr

[benjaminhuo]

Thank you @MarkusFreitag !