rtmp: support additional Enhanced-RTMP features (#3685) (#4088)

new features:
* support publishing Opus and AC-3 tracks
* support publishing more than 2 tracks. This is compatible with OBS multitrack video and OBS VOD audio track

internal/protocols/rtmp/message/msg_user_control_ping_request.go

@@ -0,0 +1,43 @@
+package message //nolint:dupl
+
+import (
+	"fmt"
+
+	"github.com/bluenviron/mediamtx/internal/protocols/rtmp/rawmessage"
+)
+
+// UserControlPingRequest is a user control message.
+type UserControlPingRequest struct {
+	ServerTime uint32
+}
+
+func (m *UserControlPingRequest) unmarshal(raw *rawmessage.Message) error {
+	if raw.ChunkStreamID != ControlChunkStreamID {
+		return fmt.Errorf("unexpected chunk stream ID")
+	}
+
+	if len(raw.Body) != 6 {
+		return fmt.Errorf("invalid body size")
+	}
+
+	m.ServerTime = uint32(raw.Body[2])<<24 | uint32(raw.Body[3])<<16 | uint32(raw.Body[4])<<8 | uint32(raw.Body[5])
+
+	return nil
+}
+
+func (m UserControlPingRequest) marshal() (*rawmessage.Message, error) {
+	buf := make([]byte, 6)
+
+	buf[0] = byte(UserControlTypePingRequest >> 8)
+	buf[1] = byte(UserControlTypePingRequest)
+	buf[2] = byte(m.ServerTime >> 24)
+	buf[3] = byte(m.ServerTime >> 16)
+	buf[4] = byte(m.ServerTime >> 8)
+	buf[5] = byte(m.ServerTime)
+
+	return &rawmessage.Message{
+		ChunkStreamID: ControlChunkStreamID,
+		Type:          uint8(TypeUserControl),
+		Body:          buf,
+	}, nil
+}