Implement mentor availability endpoint

[Shrenik0321]

Purpose

The purpose of this endpoint is for the mentor to update their availability

Goals

Approach

Screenshots

Checklist

• [x] This PR doesn't commit any keys, passwords, tokens, usernames, or other secrets.
• [ ] I have read and understood the development best practices guidelines ( http://bit.ly/sef-best-practices )
• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation

Related PRs

## Test environment ## Learning

[Shrenik0321]

I havent written the tests yet. Wanted to make sure the endpoint is fine. Will do once approved. Also im not sure about the route @anjula-sack