Add extra field to Cancel API indicating reason of cancellation

## Description
As a user,
I need to explain mentorship relation cancellation reason,
so that I can help improving the system with proper feedback.

## Acceptance Criteria
### Update
- [ ] Optional text field, e.g.: `cancellation_reason`, to API request data of Cancel API and Mentorship relation database model
- [ ] Add and fix tests for this
- [ ] Document properly on Swagger UI

## Definition of Done
- [ ] All of the required items are completed.
- [x] Approval by 1 mentor.

## Estimation
3 hours