Accept mentorship request API

[isabelcosta]

Description

• Created API to accept a mentorship request
• Created test for DAO class
• Created test for API

Fixes #12

Type of Change:

• Code

Code/Quality Assurance Only

• New feature (non-breaking change which adds functionality pre-approved by mentors)

How Has This Been Tested?

Created tests for the DAO object and ran all the tests (new and old ones)
Tested on Swagger UI, with 3 different users, and a mentorship request sent by user 1 to user 2.

Checklist:

• My PR follows the style guidelines of this project
• I have performed a self-review of my own code or materials
• I have commented my code or provided relevant documentation, particularly in hard-to-understand areas
• Update Swagger documentation and the exported file at /docs folder

Code/Quality Assurance Only

• My changes generate no new warnings
• My PR currently breaks something (fix or feature that would cause existing functionality to not work as expected)
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes