Update electron statuses for `SSHExecutor`

[kessler-frost]

Once separation of workflow and electron statuses is done, the electron level statuses need to be updated to accommodate executor dependent statuses. In this case the following status definitions will be updated:

• REGISTERING - Connection established to the remote machine and files are being transferred

• RUNNING - Execution invoked on the remote machine

• COMPLETING - Results files are being retrieved, temporary files are being deleted, and the connection is being closed

The classes for these statuses will need to be created similar to status classes defined in covalent/_shared_files/statuses.py. Then, in order to save the status, one can do:

...

status_store = task_metadata["status_store"]
status_store.save(CustomStatus())

...

This will propagate the status updation to the DB.

Acceptance Criteria:

• Above mentioned statuses need to be updated inside the local executor
• Tests need to be added to verify if those definitions are as expected