Rest Client Reactive: Provide custom ObjectMapper in request scope

[Sgitario]

With these changes, we can now register custom object mappers when creating a client programmatically:

clientAllowsUnknown = RestClientBuilder.newBuilder()
                .baseUrl(new URL(wireMockServer.baseUrl()))
                .register(ClientObjectMapperUnknown.class)
                .build(MyClient.class);

where ClientObjectMapperUnknown is:

public static class ClientObjectMapperUnknown implements ContextResolver<ObjectMapper> {
        @Override
        public ObjectMapper getContext(Class<?> type) {
            return new ObjectMapper()
                    .disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
                    .disable(SerializationFeature.FAIL_ON_EMPTY_BEANS);
        }
    }

I implemented this feature by injecting the rest client context via the interface ClientRestHandler. Then, the rest client context has registered all the context resolvers, so the jackson message reader/writer can get the custom object mappers.

Fix #26152
Relates #26988