Add the unstable MSC3814 dehydrated-device endpoints to both MDAD
worker-routing models:

- the specialized client_reader bucket
- the broad generic_worker route list

This is not a docs-driven change. Current workers.md does not meaningfully
spell out these paths, but the current Synapse code does mount them via the
normal devices servlet registration path, and non-main client workers do not
skip that servlet group.

That makes these endpoints a good fit for the same worker buckets that already
handle the surrounding device- and E2EE-related client APIs.

Refs:
- b99a58719b/docs/workers.md (synapseappgeneric_worker)
- b99a58719b/synapse/rest/client/devices.py (L256-L459)
- b99a58719b/synapse/rest/__init__.py (L81-L129)
- b99a58719b/synapse/rest/__init__.py (L179-L197)