arrow

GH-44066: [Python] Add Python wrapper for JsonExtensionType (#44070)

### Rationale for this change

We [added canonical JsonExtensionType](https://github.com/apache/arrow/pull/13901) and we should make it usable from Python.

### What changes are included in this PR?

Python wrapper for `JsonExtensionType` and `JsonArray` are added on Python side as well as `JsonArray` on c++ side.

### Are these changes tested?

Python tests for the extension type and array are included.

### Are there any user-facing changes?

This adds a json canonical extension type to pyarrow.
* GitHub Issue: #44066

Lead-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>