community/pipewire: add custom polkit rule

fixes #13432 (closed)

The underlying issue is that rtkit will only grant real time priority if the calling process belongs to an active session. This might not be the case in case of headless single board computers used as standalone audio player. In addition, polkit cannot determine active sessions on session managers other than (e)logind.

The added rules grants every user in the audio group access to rtkit, preventing audio stuttering with pipewire for both users of e.g. seatd and headless audio players.

@StacyHarper: This basically is a copy-pasting the fix you suggested in #13432 (comment 206352) but uses the audio group instead of plugdev.