Make ProblemContext fields public

[chberger]

This PR fixes #301. Without public access to ProblemContext data it's almost useless to implement custom ProblemPostProcessors. In addition, this PR did some reordering of modifiers as at a first glance it looked like the register and rest methods of PostProcessorsRegistry would also not be public.

[lwitkowski]

@chberger thank you, I really appreciate your contribution. Before I merge this, let me think about how those processors are registered - I never planned to make it possible to implement custom processors, so ExceptionMapperBase.postProcessorsRegistry.register was kind of okey-ish as internal mechanism, but not very elegant for application developers to interact with.

I'll try to make it work with CDI, so that you can simply create a processor, annotate it with @ApplicationScoped and it would be picked up by the extension automagically.

[chberger]

@lwitkowski This sounds great! It's exactly what we're looking for (see #301 (comment)). Please let me know if you need help, because we would need the possibility of custom post-processors urgently.

[lwitkowski]

@chberger Please address one small comment, then I'll merge it.

I have a working prototype with CDI, nevertheless I will do it in a separate PR with code changes and documentation on how to use it and will ask you for a review. WDYT?

[chberger]

@lwitkowski Sure, I'd love to do a review - thank you!