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

[kou]

I'll merge this in a few days if nobody objects this.

[llama90]

@jorisvandenbossche Hello. Would you take a look at this PR when you get a chance? Thanks!

[kou]

Could you rebase on main to fix CI failures?

I'll merge this in the next week if nobody objects this.

[llama90]

Hello @kou! Will this PR need more time before it can be merged?

Thank you as always!

[llama90]

@jorisvandenbossche Fine! I applied your suggestions and fixed the unit test.

Thank you for review :)

[conbench-apache-arrow[bot]]

After merging your PR, Conbench analyzed the 7 benchmarking runs that have been run so far on merge-commit e254c43c095bd6e33d07129257e11760f885f299.

There were no benchmark performance regressions. 🎉

The full Conbench report has more details. It also includes information about 5 possible false positives for unstable benchmarks that are known to sometimes produce them.