Adding an endpoint to get all categories created by admin

[LarisaTerenteva]

Is your feature request related to a problem? Please describe.
It is now only possible to get the separate category by id or the list of all existing categories, which is not quite comfortable while managing categories by admin user, when there are more than one user in a system with admin role.

Describe the solution you'd like
I propose adding a new endpoint by analogy with courses so it would be possible to manage categories created by the separate admin while being logged in to this admin account:
GET http://qa-school-test.quantori.academy/api/v1/categories/mine

[viraxslot]

I think we can make it even more flexible:

1. Add the possibility to get a list of admins
2. Add the possibility to get categories by admin id

I'll think about it.