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.