Configure RSocket support in GraphQL

[bclozel]

Since spring-projects/spring-graphql#339, Spring for GraphQL also allows RSocket transport.
Spring Boot should also support this by auto-configuring a Controller that, using RSocketGraphQlHandler, handles RSocket messages at a configured path.

[rstoyanchev]

Note there is also an RSocketGraphQlClient and RSocketGraphQlTester which use RSocketRequester.

Currently, if you start with RSocketGraphQlClient.builder(), it will create a new RSocketRequester.Builder and set the dataMimeType to "application/graphql+json", but if you start with builder(RSocketRequester.Builder), which could be the builder from Boot's RSocket starter, then you'd have to make that builder(RSocketRequester.Builder).dataMimeType("application/graphql+json").

Either the Boot start should provide an RSocketRequester.Builder for GraphQL or we could set the dataMimeType even when we are given a pre-configured builder instance. In any case, if there isn't anything to be added on the Boot side for the client or tester, a short update in the documentation with guidance would also make sense.