arrow

GH-36038: [Python] Implement __reduce__ on ExtensionType class (#36170)

### Rationale for this change
`ExtensionType` subclasses can't be pickled if `__reduce__` method isn't implemented separately.

### What changes are included in this PR?
Add `__reduce__` method to `ExtensionType` class.

### Are these changes tested?
Yes, test is added to python/pyarrow/tests/test_extension_type.py.

### Are there any user-facing changes?
No.
* Closes: #36038

Authored-by: AlenkaF <frim.alenka@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>