Integration with Jakarta Concurrency 3.0

I would like to suggest that we add a new section under [section 11.2 Integrations with Jakarta EE Technologies](https://jakarta.ee/specifications/restful-ws/3.0/jakarta-restful-ws-spec-3.0.html#javaee) for Jakarta Concurrency 3.0. In particular, I think that when a user passes a `ManagedExecutorService` to the `ClientBuilder.executorService(...)` method, that we ensure that implementations return `CompletionStage` objects using the new methods added to Concurrency 3.0 in issue https://github.com/eclipse-ee4j/concurrency-api/issues/40

This would resolve the nondeterministic problems listed in the issue and make it portable - at least for environments that support Jakarta Concurrency 3.0.

Start by reading section 11.2 of the Jakarta RESTful Web Services 3.0 specification and the referenced Jakarta Concurrency issue 40. Determine how ClientBuilder.executorService(...) and ManagedExecutorService should interact with CompletionStage methods, then document the portable integration behavior for Jakarta Concurrency 3.0 environments.

Written by the indexing model from the issue text.