Peeking via MSC2753 (#1370)

Initial implementation of MSC2753, as tested by https://github.com/matrix-org/sytest/pull/944.
Doesn't yet handle unpeeks, peeked EDUs, or history viz changing during a peek - these will follow.
https://github.com/matrix-org/dendrite/pull/1370 has full details.

roomserver/internal/api.go

@@ -22,6 +22,7 @@ type RoomserverInternalAPI struct {
 	*query.Queryer
 	*perform.Inviter
 	*perform.Joiner
+	*perform.Peeker
 	*perform.Leaver
 	*perform.Publisher
 	*perform.Backfiller
@@ -83,6 +84,13 @@ func (r *RoomserverInternalAPI) SetFederationSenderAPI(fsAPI fsAPI.FederationSen
 		FSAPI:      r.fsAPI,
 		Inputer:    r.Inputer,
 	}
+	r.Peeker = &perform.Peeker{
+		ServerName: r.Cfg.Matrix.ServerName,
+		Cfg:        r.Cfg,
+		DB:         r.DB,
+		FSAPI:      r.fsAPI,
+		Inputer:    r.Inputer,
+	}
 	r.Leaver = &perform.Leaver{
 		Cfg:     r.Cfg,
 		DB:      r.DB,