Allow selectively enabling problem support on specific APIs / clients

[jpraet]

Continuation of #252.

An application may expose / consume multiple REST APIs.

• What if it needs to expose one API that uses problems, but also another API that doesn't?
• What if it needs to call one API that uses problems, but also another API that doesn't?

General idea is to allow to configure whether problem support should be enabled by default:

• io.github.belgif.rest.problem.server-side-enabled=true/false (default true)
• io.github.belgif.rest.problem.client-side-enabled=true/false (default true)

Combined with a way to enable/disable on API or client level, perhaps using annotations @EnableProblems @DisableProblems if feasible.