Add song composer metadata support

[keilogic]

Summary

• add a composer field to songs in Core Data model v50
• parse Ampache <composer> tags and Subsonic composer attributes
• show composer in song row/player secondary text and long song metadata/details
• cover Ampache song and playlist parsing plus Song storage behavior in tests

Refs #312 item 4.

If this is accepted as fulfilling the paid composer-field item, payment can be sent to https://www.paypal.com/paypalme/aogkft.

Verification

• git diff --check upstream/master...add-song-composer-metadata
• changed-file conflict-marker scan across the branch diff
• parsed .xccurrentversion and Amperfy v50.xcdatamodel/contents as XML with PowerShell

Not run: Swift/Xcode tests, because this Windows environment does not have swift, swiftformat, or xcodebuild installed.

[BLeeEZ]

The composer should only be visible in the song details. To display the composer in a new row is currently not possible since apple doesn't allow to use spreadsheets as views in iOS.
There are also XML tests missing for Ampache and Subsonic.