Add "request audio transcode format"

[selfhoster]

Adds menu items in Cache/Network to optionally enable DSub to specify audio transcode format alongside subsonic musicStream requests.

Included options are:

• raw
• opus
• ogg
• aac
• mp3
• m4a
• flac

This subsonic feature is documented here:

Parameter	Required	Default	Comment
format	No		(Since 1.6.0) Specifies the preferred target format (e.g., "mp3" or "flv") in case there are multiple applicable transcodings. Starting with 1.9.0 you can use the special value "raw" to disable transcoding.

Since including format is a part of the 1.6.0 (1.9.0 for raw) subsonic API spec, it is up to subsonic compatible servers that advertise 1.6.0 compliance to correctly handle this.
I have tested this feature with Funkwhale and Airsonic:

• Funkwhale handles all requested formats apart from aac and m4a correctly (will be opening a seperate issue on their gitlab for this)
• Airsonic only handles the raw format request correctly but does not break on the other formats (also a separate Airsonic issue)

This pull request should enable Funkwhale as a viable music backend for DSub, and should close:

• issue 958
• related issue on Funkwhale project

[daneren2005]

Will hopefully finally get around to really checking this out in the next week or so. Been just working my way backwards through PRs since I started working on this again.

[MoritzMaxeiner]

One thing to add: Neither ogg nor m4a really belong on this list, as they're container formats with several different possible codecs inside, see MoritzMaxeiner@434c125