Adds the option to use a pre configured RSocketRequester

[iromu]

This PR adds support to reuse a RSocketRequester object, with the purpose of reusing its load balancing features, and to avoid duplicating code in case the applciations uses GraphQL over RSocket and RSocket clients.

As described on Issue #497

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[rstoyanchev]

The challenge with accepting a fully built RSocketRequester is that it becomes mutually exclusive with all other builder methods. So you have to prepare your requester externally and bypass the builder essentially.

I think it would be better if we exposed the same as what is available on the RSocketRequester.Builder since RSocketGraphQlClient.Builder is meant to be just a convenience wrapper around it. In other words:

interface Builder<B extends Builder<B>> extends GraphQlClient.Builder<B> {

    // ...

    B transports(Publisher<List<LoadbalanceTarget>> targetPublisher, LoadbalanceStrategy loadbalanceStrategy);

}

Do you want to rework the PR along those lines, or otherwise I can also take it from here?

[iromu]

I agree it is an edge case that an application will reuse a RSocketRequester. The code works, the only problem I saw was having to set a Json Decoder by default.

Adding transports as a builder method looks more aligned with the current code. Let me update the PR to see how the code will look like.

[iromu]

It does looks better.

@Bean
public RSocketGraphQlClient rSocketGraphQlClient(RSocketGraphQlClient.Builder<?> builder,
                                                 ReactiveDiscoveryClient discoveryClient
) {
    Flux<List<LoadbalanceTarget>> servers = getServers(discoveryClient, "GRAPHQL-MONGO");
    return builder
        .transports(servers, new RoundRobinLoadbalanceStrategy())
        .route("graphql")
        .build();
}

[rstoyanchev]

Mostly good.

There is a small detail missing, that the loadbalance details will need to be passed to the DefaultRSocketGraphQlClient and then used in the mutate method, or otherwise mutate wouldn't work well if used. That's relatively minor though. I could take it from here.

[iromu]

Feel free to take it from here and apply any styling preference.