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ARROW-15639 [C++][Python] UDF Scalar Function Implementation

PR for Scalar UDF integration

This is the first phase of UDF integration to Arrow. This version only includes ScalarFunctions.
In future of PRs, Vector UDF (using Arrow VectorFunction), UDTF (user-defined table function)
and Aggregation UDFs will be integrated. This PR includes the following;

- [x] UDF Python Scalar Function registration and usage
- [x] UDF Python Scalar Function Examples
- [x] UDF Python Scalar Function test cases
- [x] UDF C++ Example extended from Compute Function Example
- [x] Added aggregation example (optional to this PR: if required can remove and push in a different PR)

Closes #12590 from vibhatha/arrow-15639

Lead-authored-by: Vibhatha Abeykoon <vibhatha@gmail.com>
Co-authored-by: Vibhatha Lakmal Abeykoon <vibhatha@users.noreply.github.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Co-authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>