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ARROW-8344: [C#] Bug-fixes to binary array plus other improvements

This PR fixes a few bugs in `BinaryArray.Builder()`:
* Fixes the `Clear()` method, which previously would break all subsequently-appended values (see JIRA ticket for examples).
* Makes the `Build()` method idempotent, by making sure the builder's internal arrays are not modified when it is called.

And makes a few other improvements:
* Comprehensive test coverage of the builder's key methods (this was written in proper TDD-style, so these tests would fail if run on `master`).
* XML docstrings for all methods affected.

Closes #7671 from mr-smidge/arrow-8344/string-array-builder-clear-bugfix

Authored-by: Adam Szmigin <adam.szmigin@jetstoneam.com>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>