Current Synapse registers the MatrixRTC transport discovery endpoint on
client-serving workers when MSC4143 is enabled, but MDAD does not model
that path in either its client-reader bucket or its broader generic-
worker endpoint list.

Add the unstable MatrixRTC transport discovery route so MDAD's worker
routing matches the current upstream worker surface for this endpoint.
This is a small, isolated routing addition for a simple authenticated
GET endpoint.

Refs:
- b99a58719b/synapse/rest/client/matrixrtc.py (L30-L52)
- b99a58719b/synapse/rest/__init__.py (L81-L129)
- b99a58719b/synapse/rest/__init__.py (L179-L197)