Search and filtering for endpoints

**Is your feature request related to a problem?**
I was making a write-up about best REST practices and something essential came to my mind. A functionality so common that it's surprising we don't have it - search.

**Describe the solution you'd like**
I'd like features listed above to be supported in all endpoints (if applicable). It could be done using [`BaseModel.filter()`](https://flask-sqlalchemy.palletsprojects.com/en/2.x/queries/#querying-records) method that Flask-SQLAlchemy provides.

### Search URL examples
`/users?username=john`
`/users?location=us`
`/mentorship_relation/{request_id}/tasks?title=learn_react`

**Describe alternatives you've considered**
We can do all this stuff on the client side and considering current scale of the project, it's not really a big deal. However, serious problems will arise if the number of users (or data in general) gets increased.