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

[llama90]

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