Add average-salary get endpoint

[elhmn]

This pull request add the GET average-salary endpoint

Steps to verify:

• move to the backend folder with cd ./backend
• run make start-postrges to run an initialized database
• run make run to launch the api
• then run the command curl -X GET localhost:7000/average-rating you should see something like this

{"rating":3,"salary":2543437}

[Asam237]

[elhmn]

try with these average-rating

[Asam237]

Its good