Add a full-codecs extension

Adds a ffmpeg-full-codecs extension with a good selection of codecs. This was requested so that apps can mostly stop rebuilding ffmpeg in order to get additional codecs, although there will be some edge cases where the selection provided here is not enough, I expect.

Additionally reworks the configuration for ffmpeg into something clearer and more maintainable. I define variables for enabling encoders, decoders, etc. which allows this to be extended in our extension rebuilds, without repeating things.

A bunch more free codecs are enabled in the core ffmpeg in the runtime, so we provide a more featureful ffmpeg even without the extensions.

These extensions are technically SDK extensions, and it may be worth renaming them as such, however this allows app-level opt-in of the codecs shipped.

We should definitely document this change.

Closes #599 (closed)