Add public API to return global headers for inclusion in containers

[tmatth]

We currenlty don't have an equivalent to aom_codec_get_global_headers() which is implemented as
https://aomedia.googlesource.com/aom/+/f8d6a1653476662a7b00a1564afe37516adeba1a/av1/encoder/encoder.c#6402

See:
https://code.videolan.org/videolan/vlc/-/merge_requests/6361
https://aomedia.issues.chromium.org/issues/42301044
https://aomedia.issues.chromium.org/issues/379430935