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GH-37669: [C++][Python] Fix casting to extension type with fixed size list storage type (#42219)

### Rationale for this change

Casting to an extension type with fixed-size list storage type was segfaulting. The underlying issue was a debug check in the casting kernel in the code path about pre-allocated data, but in this case we shouldn't be pre-allocating anything, because "cast to extension type" _can_ be zero copy, and we should let that be handled by the underlying cast to the storage type.

### What changes are included in this PR?

Specifically mark the cast kernels to extension type as `NullHandling::COMPUTED_NO_PREALLOCATE` and `MemAllocation::NO_PREALLOCATE`

### Are these changes tested?

Yes

### Are there any user-facing changes?
No
* GitHub Issue: #37669

Authored-by: Joris Van den Bossche <jorisvandenbossche@gmail.com>
Signed-off-by: Will Jones <willjones127@gmail.com>