avtp: Introduce AVTP source element

avtp: Introduce AVTP source element

This patch introduces the avtpsrc element which implements a typical
network source. The avtpsrc element receives AVTPDUs encapsulated into
Ethernet frames and push them downstream in the GStreamer pipeline.
Implementation if pretty straightforward since the burden is implemented
by GstPushSrc class.

Likewise the avtpsink element, applications that utilize this element
must have CAP_NET_RAW capability since it is required by Linux to open
sockets from AF_PACKET domain.