Employee API: added supervisor_id field

Added the feature to set/update Employee's Supervisors.

We just added the feature to set/remove Supervisors using the Public API on the POST and PATCH endpoints.

Include the supervisor_id in the request body with the Employee ID value of the supervisor you want to assign.

Notes:

• If the supervisor_id field is not in the request body, the value will not be affected.
• If you want to remove the supervisor of the employee, provide "supervisor_id": null.
• If the employee to be assigned as supervisor does not exist, an error response with status 422 will be returned.
• If the supervisor is the same employee that is being updated, an error response with status 422 will be returned.

For more details, check the documentation on how to create or update an employee using the Public API.