RestClient Gson usage

[rufus20145]

It would be great, if RestClient followed the spring.mvc.converters.preferred-json-mapper=gson setting or had a similar own. Now it only checks the classpath for the existence of jackson classes (in DefaultRestClientBuilder.initMessageConverters()). Since my application includes spring-boot-starter-actuator jackson will always be on the classpath, so there is no way RestClient will create GsonHttpMessageConverter instead of MappingJackson2HttpMessageConverter. As a workaround we can pass HttpMessageConverters from application context to RestClient.Builder, but DefaultRestClientBuilder.initMessageConverters() creates more converters, than I have in context. It seems, I don't need them, but I haven't tested it properly.

[wilkinsona]

This is related to #33870 (comment) where we were also wondering about a property for choosing Gson that isn't specific to MVC.