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GH-41389: [Python] Expose byte_width and bit_width of ExtensionType in terms of the storage type (#41413)



### Rationale for this change

This update aligns the Python API with Arrow C++ by exposing the actual byte and bit widths of extension types from their storage type.

### What changes are included in this PR?

- Expose byte_width and bit_width properties for ExtensionType in Python, reflecting the underlying storage type.
- Add  unit tests to verify these properties

### Are these changes tested?

Yes

### Are there any user-facing changes?

Yes

* GitHub Issue: #41389

Lead-authored-by: Hyunseok Seo <hsseo0501@gmail.com>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>