[C++] Add StopToken to ExecNode

[asfimport]

Would allow things to get promptly cleaned up on Ctrl-C.

Reporter: David Li / @lidavidm

Related issues:

• [C++] Query engine umbrella issue (is a child of)

_{Note: This issue was originally created as ARROW-13297. Please see the migration documentation for further details.}

[github-actions]

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