feat: Support for composer tag

[s-fleck]

While composer (and possibly performer) tags are really obscure in most genres of music, they are pretty essential to keep a classical music collection organized IMHO. Support for these is lackluster, but as far as i could find the opensubsonic API spec includes it (see links below)

https://opensubsonic.netlify.app/docs/responses/child/
https://opensubsonic.netlify.app/docs/responses/artistid3/

Anyways if this would be easy it would be great if it could be implemented so that clients could make use of it :) If gonic already delivers this data and i wasn't aware of it, shame on me :(