Don't use supplyAsync without an executor

[phillip-kruger]

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As mentioned by @cescoffier - in server/implementation/src/main/java/io/smallrye/graphql/spi/DataFetcherService.java

"Don't use supplyAsync without an executor. It picks a thread from the fork/join pool, you don't have any control."

That sounds like we might need some sort of config mechanism for users to be able to specify the executor service they want to use.

[cescoffier]

I'm not sure it's a user setting. What about an infrastructure setting. For example in Quarkus, use the quarkus worker pool.