add audio encoder frame duration configure parameter

[bdrtc]

we use webcodecs audio encoder encode opus realtime and send via webtransport, but find its frame duration（ptime） is 60ms by default and no parameter to configure it.
60ms is not acceptable for realtime stream, it add extra 40ms fix delay compare to webrtc's 20ms ptime, hope the configure for audio ptime to encoder parameter

[sandersdan]

Agreed, there should probably be a configurable parameter for packet duration.

[bdrtc]

close this as solved by #551