event: add new GST_EVENT_PROTECTION

event: add new GST_EVENT_PROTECTION

In order for a decrypter element to decrypt media protected using a
specific protection system, it first needs all the protection system
specific  information necessary (E.g. information on how to acquire
the decryption keys) for that stream.

The GST_EVENT_PROTECTION defined in this commit enables this information
to be passed from elements that extract it (e.g. qtdemux, dashdemux) to
elements that use it (E.g. a decrypter element).

API: GST_EVENT_PROTECTION
API: gst_event_new_protection()
API: gst_event_parse_protection()

https://bugzilla.gnome.org/show_bug.cgi?id=705991