Support additional conversions in audioconvert via sndfile

[MasonRemaley]

Pipewire optionally uses sndfile for additional audio format conversions. Supporting this would involve first porting sndfile to the Zig build system.

This isn't high priority for me right now, contributions are welcome though! The steps at a very high level would be:

• Create a separate project porting sndfile to Zig's build system
• Define HAVE_SNDFILE when building audioconvert and add it as a dependency

(related to #7)