Add support for SOCKS5 proxies

[avivace]

Currently, only HTTP proxy are supported and there's no way (to my knowledge) to make spotifyd work over an SSH proxy. Workaround also don't work, see #1241

[eladyn]

Unfortunately, the different libraries we rely on don't support different proxy kinds. In particular, the proxy library that librespot uses, is quite restrictive in the types it can handle, IIRC. So we should probably work on the workaround issues, since we won't be able to easily fix our dependencies.

[eladyn]

I'm gonna convert this into a discussion, since it doesn't really seem actionable from our side.