arrow

GH-24868: [C++] Add a Tensor logical value type with varying dimensions, implemented using ExtensionType (#37166)

### Rationale for this change

For use cases where underlying datatype and number of dimensions in tensors are equal but not the actual shape we want to add a `VariableShapeTensorType`.
See https://github.com/apache/arrow/issues/24868 and https://github.com/huggingface/datasets/issues/5272

### What changes are included in this PR?

This introduces definition of `arrow.variable_shape_tensor` extension and it's C++ implementation and a Python wrapper.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

This introduces new extension type to the user.
* Closes: #24868

Lead-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>