feign.jackson.jr.JacksonJrDecoder#findTransformer is not extendable

[yvasyliev]

Description

This is a follow up of #2296.

feign.jackson.jr.JacksonJrDecoder#findTransformer is private static which makes feign.jackson.jr.JacksonJrDecoder hard to extend.

Suggestion

To migrate feign.jackson.jr.JacksonJrDecoder#findTransformer from private static to protected, so JacksonJrDecoder will be more configurable.

Also, this implementation will make #2296 less severe.