arrow

GH-20213: [C++] Implement cast to/from halffloat (#40067)



### Rationale for this change

### What changes are included in this PR?

This PR implements casting to and from float16 types using the vendored float16 library included in arrow at `cpp/arrrow/util/float16.*`.

### Are these changes tested?

Unit tests are included in this PR.

### Are there any user-facing changes?

In that casts to and from float16 will now work, yes.

* Closes: #20213

### TODO

- [x] Add casts to/from float64.
- [x] String <-> float16 casts.
- [x] Integer <-> float16 casts.
- [x] Tests.
- [x] Update https://github.com/apache/arrow/blob/main/docs/source/status.rst about half float.
- [x] Rebase.
- [x] Run clang format over this PR.
* GitHub Issue: #20213

Authored-by: Clif Houck <me@clifhouck.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>