Support writing arbitrary key/value pairs to container metadata

[Michael-Equi]

Desired Behavior

Currently only the title is allowed to be written to a containers metadata. The behavior is tested in the following unit test:

PyAV/tests/test_encode.py

Line 74 in 86f66ac

assert input_.metadata.get("key") is None

I have confirmed ffmpeg does support this by using the following CLI command given the path to a libx264 encoded video.

subprocess.run(
        [
            "ffmpeg",
            "-y",  # Overwrite output files without asking
            "-i",
            input,
            "-c",
            "copy",
            "-copyts",  # Copy timestamps from input to output
            "-metadata",
            f"key={value}",
            "-movflags",
            "+use_metadata_tags",
            output,
        ],

Example API

Allow any arbitrary key/value pair to be written to the metadata in the same way title can be written to the metadata.