srt: Add `packetfilter` property. Allows configuring FEC

FEC (Forward Error Correction) sends some added redundant data to allow the SRT receiver to reconstruct missing packets (saving retransmissions, thus improving latency at the cost of a configurable bandwidth overhead).

See: https://github.com/Haivision/srt/blob/master/docs/packet-filtering-and-fec.md

For the user to be able to configure FEC, gstreamer just needs to pass the packetfilter configuration string through to the SRT library.