opusenc: Allow OPUS_APPLICATION_RESTRICTED_LOWDELAY

Currently, default and voip are the only two options allowed for the audio-type property of opusenc. But opus actually has a third option called OPUS_APPLICATION_RESTRICTED_LOWDELAY:

https://github.com/xiph/opus/blob/4f4b11c2398e96134dc62ee794bfe33ecd6e9bd2/include/opus_defines.h#L199

It would be nice to have this added to the list of options.