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GH-20086: [C++] Cast between fixed size and variable size lists (#37292)

### Rationale for this change

These list types should be able to cast to each other. In our code base, we find users might naively make an list array (for example, with `pa.Table.from_pylist()`) and want it automatically casted to a tensor array (which is based on a fixed size list).

### What changes are included in this PR?

Adds casts from FSL -> (Large)List and (Large)List -> FSL.

### Are these changes tested?

The kernels are tested.

### Are there any user-facing changes?

Just a new kernel.
* Closes: #20086

Authored-by: Will Jones <willjones127@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>