Create API endpoint for Client

[kehwhy]

Create a client with the POST endpoint

Domain model: https://drive.google.com/drive/u/0/folders/1YTa_4vbxaqxf1G4uNzUNhzZErjqFhNG_

An example of the REST endpoints - https://github.com/hack4impact-mcgill/mu-crm-tool/blob/main/backend/app/project/routes.py

More info on REST api endpoints: https://www.restapitutorial.com/lessons/httpmethods.html