Mock auth check

[kzalys]

For the Auth System V2 project.

While the URI matcher is being worked on as part of #65 it would be useful to have some kind of auth system set up to test the v2 API as it is being implemented.

We can create a mock auth check operation which would simply check if the given token is valid and would not check if the token has access to the requested resources.

The specification for the API operation to be implemented can be found here: https://documenter.getpostman.com/view/2489646/T1DjjKP6?version=latest#db66a458-c8c0-497a-833c-1ffcff8dd141

Definition of done:

• Approved and merged PR with implementation of the mock auth check and an example of using the auth check.