Add a generic mechanism for exposing bridges' HTTP API (the provisioning
API, etc.) publicly on the Matrix domain, so tools like mautrix-manager
(https://github.com/mautrix/manager) can drive bridge login.

- Introduce global matrix_bridges_exposure_* vars (on by default),
  exposing each supported bridge under `<matrix-fqn>/bridges/<bridge>`.
- mautrix-gmessages: make the provisioning shared secret configurable
  (auto-generated in group_vars) so the provisioning API is enabled,
  route the whole bridge HTTP port via Traefik, and populate
  appservice.public_address.

Requests are authenticated by the bridge itself (per-user Matrix access
token for the provisioning API, homeserver token for the appservice
endpoints), not by the reverse proxy.

This is the first bridge converted; the other mautrix bridges will follow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>