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GH-33801: [Python] Expose C++ ExtensionTypes/ExtensionArrays in pyarrow (#33802)

* Closes #33801 

### Rationale for this change

C++ Extension Types are not correctly exposed in pyarrow

### What changes are included in this PR?

`__arrow_ext_class__` and `__arrow_ext_scalar_class__` have been moved from `ExtensionType` to `BaseExtensionType` in types.pxi.

### Are these changes tested?

Yes, a test has been added to `test_cython.py`. There may be better locations for this, but the existing cython testing machinery here is useful for generating a C++ extension type on the fly.

### Are there any user-facing changes?

I don't believe there are any user-facing changes as `__arrow_ext_class__` and `__arrow_ext_scalar_class__` are moved into a base class.

Authored-by: Simon Perkins <simon.perkins@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>