Improve time_base support & additions for ffmpeg 5-6

[encounter]

This adds various accessors and setters that are useful when building a timebase-aware transcoding pipeline.

Each commit has details on the exact changes. Happy to drop or rework any of them.

Important changes for ffmpeg 5-6 support:

• Context::new_with_codec (see also #51, #129, #149). This is important to initialize a context with the default values from a Codec.
• Output::add_stream_with: Calls avcodec_parameters_from_context, important for setting the codec parameters for a Stream. (See #118)

[Polochon-street]

thanks a lot for all these goodies, merging :)