buffer: Add GstReferenceTimestampMeta

buffer: Add GstReferenceTimestampMeta

This is a meta that generically allows to attach additional reference
timestamps to a buffer, that don't have to relate to the pipeline clock
in any way.

Examples of this could be an NTP timestamp when the media was captured,
a frame counter on the capture side or the (local) UNIX timestamp when
the media was captured.

https://bugzilla.gnome.org/show_bug.cgi?id=779213