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GH-44651: [Python] Allow from_buffers to work with StringView on Python (#44701)

### Rationale for this change

Currently `from_buffers` is not working with StringView on Python because we validate against num_buffers. This only take into account the mandatory buffers but does not take into account the variadic_spec that can be present for both string_view and binary_view

### What changes are included in this PR?

Take into account whether the type contains a variadic_spec for the non-mandatory buffers and only check lower_bound number of buffers.

### Are these changes tested?

Yes, I've added a couple of tests.

### Are there any user-facing changes?

We are exposing a new method on the Python DataType. `has_variadic_buffers` which tells us whether the number of buffers expected is only lower-bounded by num_buffers.
* GitHub Issue: #44651

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Raúl Cumplido <raulcumplido@gmail.com>