Add matrix-rustpush-bridge (iMessage)

Add the matrix-rustpush-bridge role, a Matrix <-> iMessage bridge built
on the mautrix-go bridgev2 framework using RustPush (OpenBubbles backend).

Unlike the existing mautrix-imessage/wsproxy bridge, it talks directly to
Apple's push notification service, so it needs neither a running Mac nor a
wsproxy on the homeserver. Each user supplies a hardware key extracted from a
Mac through the bridge bot's login flow.

The bridge uses its own bot username and puppet namespace (rustpushbot,
rustpush_*) so it does not collide with the wsproxy iMessage bridge.

This bridge is in early development and may have stability issues.

setup.yml

@@ -71,6 +71,7 @@
     - custom/matrix-bridge-mautrix-discord
     - custom/matrix-bridge-mautrix-slack
     - custom/matrix-bridge-mautrix-bluesky
+    - custom/matrix-bridge-rustpush
     - custom/matrix-bridge-mx-puppet-groupme
     - custom/matrix-bridge-mx-puppet-steam
     - custom/matrix-bridge-postmoogle