ObjectMapper reuse & config

Enables ObjectMapper reuse. ObjectMapper can be accessed & configured, with a convenience module registration method.

Additionally, added a per-request method which allows overriding the ObjectMapper for a specific request

It's inefficient to make new ObjectMappers with every request and incur annotation processing overhead etc.

I'm by no means a Scala expert, I'd love some feedback on where this could be taken. I'm especially interested in sharing an ObjectMapper to an entity decoding function.