Add JWT unit tests

[kjosh]

I wrote some unit tests for the JWT vulnerabilities (#398), testing the controller functions and some validator exploits to the best of my ability.

I refactored some relatively minor things to make the controller easier to test:

• created a method to create a token with a JWK header in the existing IJWTTokenGenerator, so I can also use it in the test without too much duplication
• removed the static initialization logic in JWTAlgorithmKMS, manage it's singleton lifecycle via Spring instead, mainly so I can inject it into the controller and spy on it in the test
• replace the String type of the RequestEntity parameters with Void, this should be the correct type for GET-Requests and allows creation of test objects

[preetkaran20]

Thanks a lot for the PR. PR looks good to me. merging it.

[preetkaran20]

Thank you for making this class unit testable. !!!