Support custom codec

[tongyuantongyu]

Expose codec interface in a new header codec.h so custom codecs can be registered and used by libavif.

I made this while building my own tool that performs batched hardware encoding using a long-living GPU encode session. At its best it may be added to contrib/ after some more polishing, but it's definitely too niche to be accepted as a built-in codec.

This PR allows anyone to add another codec into libavif without hacking into the library code.

[tongyuantongyu]

@y-guyon I saw your PR #2381. I think allowing users to implement their own codec is better than adding callback.

​+ @wantehchang @maryla-uc Asking if we want this feature. We probably need to refactor the codec interface to be forward-compatible and versioned, so I want to hear your opinion before working on this further.

[y-guyon]

I think allowing users to implement their own codec is better than adding callback.

Could you describe what is better exactly? I agree it probably is more complete and powerful, but it also exposes more internal stuff in the public API. I am fine with both approaches, I just wonder if the work is worth it overall. Also maybe CrabbyAvif is a more future-looking location for such a feature.

[tongyuantongyu]

Could you describe what is better exactly?

customEncodeImageFunc and customEncodeFinishFunc are basically encodeImage and encodeFinish that built-in codecs would implement and replace them when present. Yet the user still have to select a built-in codec, let its avifCodecCreate* and destroyInternal be called without doing anything. It's confusing and wasteful, and more importantly, it forces developers to ship at least one built-in codec, that will never be used, in order to use their custom codec.