Introduce GraphQL response media type in spring-graphql

[bclozel]

Spring Framework recently reflected an official policy for MediaType enum values in spring-projects/spring-framework#33754.

The MediaType APPLICATION_GRAPHQL_RESPONSE does not qualify here and we already had to manage deprecations and transitions between MediaType APPLICATION_GRAPHQL/MediaType APPLICATION_GRAPHQL_RESPONSE for Spring Framework and Spring for GraphQL.

We should consider introducing and maintaining our own media types like GraphQlMediaType.RESPONSE in spring-graphql directly. Right now our transports are located in org.springframework.graphql.server and org.springframework.graphql.client, wich no obvious location for a shared component.