Add udev rules and a profile for Sennheiser GSX devices.

This MR adds a profile and a USB rule for the Sennheiser GSX1000 and GSX1200 headphone amplifiers: https://en-ca.sennheiser.com/audio-amplifier-gsx-1000

This is based off of https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/1821494 but modified slightly based on what worked on my system (mainly, I think, by fixing a typo).

This is my first contribution to pulseaudio and indeed my first time writing any pulseaudio configuration, and I have found it woefully underdocumented. I freely admit I have no idea what I'm doing here and welcome feedback; however, I have tested this configuration on Ubuntu 20.04 with my GSX 1000 device and it works for me.

The GSX devices work somewhat like the devices under usb-gaming-headset.conf, with the main exception of exposing a 7.1 surround output sink, instead of a stereo sink.

My desired goal was to be able to direct different applications to the mono sink and to the 7.1 sink. With this configuration file, pavucontrol was able to do so, and pacmd list-sinks showed separate sinks.