Make setMid of Transceiver public

in some case like session migration from one sfu node
to another, we need manual set mid of transceiver instead of
auto generate, to make mid consistent between to sfu node.

AUTHORS.txt

@@ -86,6 +86,7 @@ Konstantin Itskov <[email protected]>
 krishna chiatanya <[email protected]>
 Kuzmin Vladimir <[email protected]>
 lawl <[email protected]>
+Len <[email protected]>
 Lukas Herman <[email protected]>
 Luke <[email protected]>
 Luke Curley <[email protected]>

peerconnection.go

@@ -660,7 +660,7 @@ func (pc *PeerConnection) CreateOffer(options *OfferOptions) (SessionDescription
 					continue
 				}
 				pc.greaterMid++
-				err = t.setMid(strconv.Itoa(pc.greaterMid))
+				err = t.SetMid(strconv.Itoa(pc.greaterMid))
 				if err != nil {
 					return SessionDescription{}, err
 				}
@@ -1092,7 +1092,7 @@ func (pc *PeerConnection) SetRemoteDescription(desc SessionDescription) error {
 			}
 
 			if t.Mid() == "" {
-				if err := t.setMid(midValue); err != nil {
+				if err := t.SetMid(midValue); err != nil {
 					return err
 				}
 			}

rtptransceiver.go

@@ -1,3 +1,4 @@
+//go:build !js
 // +build !js
 
 package webrtc
@@ -115,8 +116,8 @@ func (t *RTPTransceiver) Receiver() *RTPReceiver {
 	return nil
 }
 
-// setMid sets the RTPTransceiver's mid. If it was already set, will return an error.
-func (t *RTPTransceiver) setMid(mid string) error {
+// SetMid sets the RTPTransceiver's mid. If it was already set, will return an error.
+func (t *RTPTransceiver) SetMid(mid string) error {
 	if currentMid := t.Mid(); currentMid != "" {
 		return fmt.Errorf("%w: %s to %s", errRTPTransceiverCannotChangeMid, currentMid, mid)
 	}