[C++] Add ExecNode to arrow/compute/api.h

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Current, if I want to do anything with exec plans I have to have:

#include <arrow/compute/exec/exec_plan.h>

This should not be required if I already have:

#include <arrow/compute/api.h>

Note, when ARROW-15238 is finished the correct file might be something like arrow/engine/api.h instead of arrow/compute/api.h

Reporter: Weston Pace / @westonpace

_{Note: This issue was originally created as ARROW-15263. Please see the migration documentation for further details.}

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Vibhatha Lakmal Abeykoon / @vibhatha:
@westonpace  In ARROW-15091, you added some documents. Did it resovle the matter or this is external to that change? 

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Weston Pace / @westonpace:
@vibhatha No, that was not sufficient to resolve this issue.

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Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.