Allow enabling the Rust Postgres driver in the Complement image

Emit `use_rust_driver: true` in the generated homeserver config when
`SYNAPSE_USE_RUST_DRIVER=true` is set (default off, so the psycopg2 path is
unchanged). Passed through Complement as `PASS_SYNAPSE_USE_RUST_DRIVER=true`,
this lets the Complement suite run against the native Rust backend.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01W3G4M92AmwSSZCbmtMJU3d

docker/conf/homeserver.yaml

@@ -78,6 +78,9 @@ database:
 {% endif %}
     cp_min: {{ POSTGRES_CP_MIN or 5 }}
     cp_max: {{ POSTGRES_CP_MAX or 10 }}
+{% if SYNAPSE_USE_RUST_DRIVER == "true" %}
+  use_rust_driver: true
+{% endif %}
 {% else %}
 database:
   name: "sqlite3"