arrow

GH-34837: [GLib][Ruby] Add Arrow::{Sparse,Dense}UnionArray#get_value (#34838)

### Rationale for this change

They are missing.

### What changes are included in this PR?

The following GLib APIs are added for them:

* `garrow_union_array_get_type_code()`
* `garrow_union_array_get_child_id()`
* `garrow_dense_union_array_get_value_offset()`

`Arrow::{Sparse,Dense}UnionArray#{to_records,values}` have a backward incompatible change:

* Don't include field name:
  * Before: `{"field_name" => value}`
  * After: `value`

Because I think that `"field_name"` is useless for many cases. Only `value` is suitable for many cases.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.

**This PR includes breaking changes to public APIs.**

See the above description for details.

* Closes: #34837

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