pyarrow: Cache the imported classes to avoid importing them each time

[Tpt]

Which issue does this PR close?

• Closes Python: avoid importing pyarrow classes ever time #9438.

Rationale for this change

Speed up conversion by only importing pyarrow once.

What changes are included in this PR?

• Use PyOnceLock::import to import the types.
• Remove some not useful .extract::<PyBackedStr>()? (the Display implementation already does something similar)

Are these changes tested?

Covered by existing tests. It would be nice to add benchmark but it might require to:

• either add a dependency to a python benchmark runner
• write some hacky code to import pyarrow from criterion tests (likely by running pip/uv from the Rust benchmark code)

Are there any user-facing changes?

No