webrtcsrc: change the producer-id type for request-encoded-filter
requested to merge tkanakamalla/gst-plugins-rs:webrtcsrc-request-encoder-filter-producer-id into main
With !1477 (merged) the producer id used while emitting the request-encoded-filter can be a None if the msid of the webrtcbin's pad is None. This might not affect the signal handler written in C but can panic in an existing Rust application with signal handler which can only handle valid String type as its param for the producer id.
So change the param type to Option in the signal builder for request-encoded-fiter signal
cc Jordan Yelloz and @ocrete