Rating function (to and fro)

[Ankur-Khandelwal]

Create an API endpoint to implement the rating functionality with the following functionalities:

• NGOs can rate the volunteers
• Volunteers can rate the NGOs
• The respective net(average) ratings of NGOs and Volunteers should increase or decrease based on new ratings.

Make sure that you add a rating attribute to the schemas of NGOs and Volunteers, if it's not there.