Start mirroring remote entities asynchronously.

The MIRRORED_ENTITY_URL could be a task instead of a plain property which would block on an attempt to access it. Instead of potentially blocking .init() create a task to start the mirroring as soon as the property is accessible.
The entity will mirror all the sensors from the remote entity including the SERVICE_STATE_ACTUAL value. It will change to RUNNING as soon as the remote entity is RUNNING.

The line changed a bit after testing (directly setting SERVICE_STATE_ACTUAL instead of _EXPECTED), but the question is still valid.