feat: helpers for problem repository, merge, update problem, etc

[imabp]

So far, we are doing really good with Problem Interface. One major feature that will be helpful is having different kind of helpers available for Problem.

We currently have in the working branch, a helper, toJSON which converts the problem class to json output.
We need to find out, some more  helper functions, we need for AsyncAPI tools.

merge, updateProblem are few of them.

Let's keep this issue open for discussion about helper functions.
cc: @magicmatatjahu

[magicmatatjahu]

My suggestions:

• copy - to create new Problem instance based on existing one (implementation is in progress)
• toJSON - create from Problem instance normal JSON object with data from existing Problem instance (implementation is in progress)
• is - check if Problem instance is instance of given type
• update - update Problem instance with additional properties

We can also implement merge, but I do not yet know how it could work.

[imabp]

Is there anywhere, we are merging two problems? I don't see it.
If it is required, we can then build, according to our needs.

The copy and toJSON are ready to merge here