reject_async
Description:
Reject this certificate, asynchronously.
Before calling this method, you must call add_rejection at least once, to set the reason(s) for rejection (for instance, a certificate might be both self-signed and expired).
In or after callback, you may call reject_async.end to check the result.
notify::state will also be emitted when the connection manager
      signals that the certificate has been rejected.
Parameters:
| this | a TLS certificate | 
| callback | called on success or failure | 
| user_data | user data for the callback |