Add an endpoint where users can validate their tokens

Description

Adds an endpoint where authorized users can request their information. This allows users to verify their own token.

Changes

None.

Additions

• A new endpoint
• A rule in the Spring firewall to allow anyone to access this endpoint

Test and Review

Spring makes it very difficult (if not impossible) to test endpoints with an Authorization dependency, so no test unfortunately.

To be filled in by the reviewers

• All of the methods are commented to expectation

• The methods are tested to satisfaction

• There are no unnecessary files added to the PR

• The continuous integration has no problems with the PR

• The PR is filled in as requested e.g. labels added reviews requested

• The documentation is up-to-date

• All nullable parameters are marked as such