[YouTube] Fix serialization of Videos channel tab when it is already fetched

[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. Not needed, the API changes are on code that is not a part of the API that users should use (that's the case of NewPipe).

This PR fixes the ability to serialize ReadyChannelTabListLinkHandlers for YouTube channels. It was due to the usage of the channelHeader field of YoutubeChannelExtractor, as explained in TeamNewPipe/NewPipe#11356 (comment). Even if this usage has been removed by copying the instance members into a new object, the ChannelHeader class of YoutubeChannelHelper was not serializable, so crashes would still happen if I didn't make this class serializable.

Optional as fields and parameters of methods have been removed in the related extractor code as it is not a good practice. This simplifies in some places channel info extraction code.

This PR and the issue it fixes also raise the question of the limits of the serialization system we use, both in the app and in the extractor.

Debug NewPipe APK based on app commit 035c394cf6704c0af9077c01ebfa55b810d02140 with changes from commit 422df70: app-debug.zip

Fixes TeamNewPipe/NewPipe#11356.

[Stypox]

Nice catch! I can't really reproduce the crash on NewPipe, but it totally makes sense that it might happen. Thank you for noticing and fixing!