Assign user to a service as approver

[redhood-97]

• Create an API to add a new user to a service as the approver of a particular deployment (based on the deployment Id)
• Also, need to modify the current user schema and a field which maintains a list of all the deployment Ids (references) against it.
• Mantain a flag of 'True' or 'False' if the user has approval rights of any service.