Extend REST API by endpoint to get a specific function definition

[bossenti]

Purpose

Extend the REST API by the endpoint /v2/functions/<functionId> that returns the definition of the given function.

Remarks

PR introduces (a) breaking change(s): no

PR introduces (a) deprecation(s): no

[tenthe]

The PR looks good.
I will take a look at the e2e tests as they seem to fail on all PRs.

[bossenti]

What would happen if I query by an unknown id? Would this result in an internal exception or does the framework catch the exception and return a proper API response?

[tenthe]

That is a good point. How could we test it properly?
I guess the best way would be to mock FunctionRegistrationService.INSTANCE but since it is an enum I am not sure if it is easily possible. One option would be to change the implementation and inject an instance of FunctionRegistrationService into the FunctionsResource, then it would be easier to mock and test the REST interface.