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ARROW-9888: [Rust][DataFusion] Allow ExecutionContext to be shared between threads (again)

The idea here is to share the ExecutionContext between threads (which was previously added in https://issues.apache.org/jira/browse/ARROW-9425 / https://github.com/apache/arrow/pull/7723 but has since regressed.

This PR fixes the issue and adds a test to ensure the behavior does not revert again

(Note this is based on https://github.com/apache/arrow/pull/8079 -- I will convert it to a real PR once that is merged)

Closes #8082 from alamb/alamb/ARROW-9888-shareable-execution-context

Authored-by: alamb <andrew@nerdnetworks.org>
Signed-off-by: Andy Grove <andygrove73@gmail.com>