avcodec/libaomenc: export Sequence Header and Metadata OBUs as extradata

aom_codec_get_global_headers() is not implemented as of libaom 1.0.0 for AV1, so
we're forced to extract the relevant header OBUs from the first packet and propagate
them as packet side data.

Signed-off-by: James Almer <jamrial@gmail.com>