arrow

GH-37643: [C++] Enhance arrow::Datum::ToString (#37646)



### Rationale for this change

Add print child fields for `arrow::Datum::ToString`. Because previously it just print a type.

### What changes are included in this PR?

Add detail in `arrow::Datum::ToString`

### Are these changes tested?

no

### Are there any user-facing changes?

Yes

* Closes: #37643

Authored-by: mwish <maplewish117@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>