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

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

```Java

#include
```

This should not be required if I already have:

```Java

#include
```

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](https://issues.apache.org/jira/browse/ARROW-15263) / @westonpace

_{**Note**: *This issue was originally created as [ARROW-15263](https://issues.apache.org/jira/browse/ARROW-15263). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with arrow/compute/api.h and arrow/compute/exec/exec_plan.h to compare the public API and the current ExecNode location. Check ARROW-15238 and the migration documentation before choosing between arrow/compute/api.h and arrow/engine/api.h; done when exec-plan users can access ExecNode through the intended public header without the extra include.

Written by the indexing model from the issue text.