sdp: Add support for parsing the extmap attribute from caps and storing inside caps

sdp: Add support for parsing the extmap attribute from caps and storing inside caps

The extmap attribute allows mapping RTP extension header IDs to
well-known RTP extension header specifications. See RFC8285 for details.

We store the extmap attribute either as string in the caps
  extmap-X=extensionname
where X is the integer extension header ID, or as 3-tuple of strings
  extmap-X=<direction,extensionname,extensionattributes>
where direction or extensionattributes are allowed to be the empty
string.

Both formats are allowed because usually only the extension name is
given and it's much simpler to handle in caps.