[FR] Support custom exception mappers in conjure-undertow-annotations

[aldexis]

What happened?

I am trying to make use of conjure-undertow-annotations and need exceptions to be mapped in a specific format so that it can interface with third-party code.

That format unfortunately doesn't fit the SerializableError format.

Our solution for now is to catch such errors, have our undertow interface return a union of error/response and custom-serialize that, but that feels non-ideal.

What did you want to happen?

Ideally, we would be able to indicate a custom exception serializer in the @handle annotation (which seems to be mentioned as a TODO there)