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GH-37484: [Python] Add a FixedSizeTensorScalar class (#37533)

### Rationale for this change

When working with `FixedSizeTensorArray` we want to access individual tensors. This would be enabled by adding:
```python
def FixedSizeTensorScalar(pa.ExtensionScalar):
    def to_numpy_ndarray(): ...
```

See #37484.

### What changes are included in this PR?

This adds `FixedSizeTensorScalar` and tests for it.

### Are there any user-facing changes?

Yes, when calling `FixedSizeTensorArray[i]` we would get back `FixedSizeTensorScalar` instead of `ExtensionScalar`.
* Closes: #37484

Lead-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Alenka Frim <AlenkaF@users.noreply.github.com>
Signed-off-by: Antoine Pitrou <antoine@python.org>