Bidirectional volume control (ALSA)

[Yanndroid]

Is your feature request related to a problem? Please describe.
When using "alsa" or "alsa_linear" as a volume controller, external changes to the ALSA volume are not applied in spotifyd. Instead, spotifyd overwrites the volume every time.

Describe the solution you'd like
Bidirectional volume changes. Changing the volume in ALSA (via alsamixer, for example) also changes it in spotifyd.

Describe alternatives you've considered
None.

Additional context
I vibe-coded some garbage that surprisingly works like expected, but I don't have enough knowledge in Rust to judge it. It might help to understand my feature request though. see here

[Yanndroid]

Alternatively, relative volume could also be an option, rather than overwriting the volume.
And to avoid hitting the limit at 0 or 100%, we can reset it to 50% after every change.

[ioctlLR]

This would be nice, especially if paired with "initial_volume=keep" (or something similar) that would make spotifyd pick up the current volume setting during startup.