Ensure access to configured mime types in client-side RSocket requester or responder

[rstoyanchev]

Until rsocket/rsocket-java#636, RSocketFactory supported only Function<RSocket, RSocket> for client side responders, which meant they had no access to the MimeType for the connection unless they were a) anonymous (or otherwise session scoped), and b) the MimeType was passed to them.

In the latest snapshots RSocketFactory provides access to the ConnectionSetupPayload on the client side too via a BiFunction. We can now change MessageHandlerAcceptor to get the data MimeType for the connection regardless of where it's used (client or server).

[rstoyanchev]

The above took care of the client-side responders, but not the client-side requester. For example if the application configures mime types via Consumer<RSocketFactory.ClientRSocketFactory> on RSocketRequest.Builder, the built RSocketRequester would be unaware of the change.