Add Support for Jackson JR

[ashleyfrieze]

At the moment, the JacksonEncoder and Decoder is bound to the core Jackson Databind project.

There's a lighterweight version of Jackson - https://github.com/FasterXML/jackson-jr

For use of Feign in AWS Lambdas and other smaller projects, it would be useful to have support. I'll raise a PR.