mautrix-gmessages: expose bridge HTTP API (for mautrix-manager and similar)

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>

roles/custom/matrix-bridge-mautrix-gmessages/templates/config.yaml.j2

@@ -181,7 +181,7 @@ appservice:
     address: {{ matrix_mautrix_gmessages_appservice_address }}
     # A public address that external services can use to reach this appservice.
     # This value doesn't affect the registration file.
-    public_address: https://bridge.example.com
+    public_address: {{ matrix_mautrix_gmessages_bridge_public_address | to_json }}
 
     # The hostname and port where this appservice should listen.
     # For Docker, you generally have to change the hostname to 0.0.0.0.
@@ -247,7 +247,7 @@ provisioning:
     prefix: /_matrix/provision
     # Shared secret for authentication. If set to "generate" or null, a random secret will be generated,
     # or if set to "disable", the provisioning API will be disabled.
-    shared_secret: disable
+    shared_secret: {{ matrix_mautrix_gmessages_provisioning_shared_secret | to_json }}
     # Whether to allow provisioning API requests to be authed using Matrix access tokens.
     # This follows the same rules as double puppeting to determine which server to contact to check the token,
     # which means that by default, it only works for users on the same server as the bridge.