Related to https://github.com/spring-projects/spring-framework/issues/23137
The metadata in Spring Messaging for RSockets now supports any
arbitrary objects for setup payload, including composition.
* Switch the `ClientRSocketConnector` to fully delegate to the
`RSocketRequester.Builder` inheriting possible metadata encoding/decoding
in the target `RSocketRequester` implementation
* Turn off a default `dataMimeType` from the `MimeTypeUtils.TEXT_PLAIN`
to the `null` by default relying on the encoder/decoder logic in the
target RSocket wrappers
* Expose more delegating options in the `ClientRSocketConnector`, like
`setupRouteVars`, `setupMetadata`
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