Support AV1 encoding for Pixel Streaming on Linux

[MWillWallT]

Hi there!

I’m happy to report that AV1 is well on its way, you can see AV1 support added in this commit here:
https://github.com/EpicGames/UnrealEngine/commit/d28eca2334ca246166f5769dcbcfd616ecbfa4b1