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GH-34911: [C++] Add first and last aggregator (#34912)



### Rationale for this change
This PR adds "first" and "last" aggregator and support using those with Acero's segmented aggregation.

### What changes are included in this PR?
- [x] Numeric Scalar Aggregator (bool, int types, floating types)
- [x] Numeric Hash Aggregator (bool, int types, floating types)
- [x] Docstring
- [x] Non-Numeric Scalar Aggregator (string, binary, fixed binary, temporal)
- [x] Non-Numeric Hash Aggregator (string, binary, fixed binary, temporal)
- [x] Add `ordered` flag in aggregate kernels
- [x] Implement and test skip null
- [x] Update compute.rst

### Are these changes tested?
- [x] Compute Kernel Test (Scalar Kernels, all supported datatypes)
- [x] Hash Aggregate Test (Hash Kernels, all supported datatypes)
- [x] Segmented Aggregation Test (Both Scalar and Hash Kernels)

### Are there any user-facing changes?
Yes. Added First and Last aggregator.

Authored-by: Li Jin <ice.xelloss@gmail.com>
Signed-off-by: Weston Pace <weston.pace@gmail.com>