Add client config flag to explicitly signal Opus support

[s3lph]

Add client config flag to explicitly signal Opus support even when the Opus audio codec is not loaded.

This is useful if a client based on gumble doesn't actually process audio at all. By signalling Opus support anyway, this doesn't force everyone back to CELT if a gumble client is the only client not claiming Opus support. We can't be sure that the server admin has actually set opusthreshold = 0, so this is a pure-client way to prevent a CELT fallback.

[ghost]

how about a dummy opus codec that registers itself but doesn't process the audio?

[s3lph]

That is actually what I proposed initially as a solution for matterbridge (the project we want this for, see 42wim/matterbridge#1750 for the discussion), but @42wim (matterbridge maintainer) and I agree that we find the dummy codec solution to be a rather ugly workaround for something that's a simple boolean flag. We do have a working dummy codec implementation (42wim/matterbridge#1764), but would like to avoid using it.