Delete mentorship request API

[isabelcosta]

Description

I developed the API to delete a mentorship request: DELETE /mentorship_relation/{id}
With these constraints:

• Only the requester of the relation can delete this
• The relation has to be in a pending state to be deleted

Fixes #65

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?

• Wrote tests for DAO class and the API itself.
• DAO tests covers all the predicted use cases of the delete_request function. Its tests if the request is deleted or not from the database.
• Ran these tests with success.

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
• 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

[isabelcosta]

Will fix the endpoint according to @m-murad suggestion:

DELETE /mentorship_relation/{id}

[Dilu9218]

it is a good practice to avoid comments that include "what" a code line is doing rather that "why" a code line is there.

[isabelcosta]

Thank you @Dilu9218! In the future for this case should I avoid these comments at all or does this code need a "why"?