arrow

GH-44214: [C++] JsonExtensionType equality check ignores storage type (#44215)

### Rationale for this change

As noted in https://github.com/apache/arrow/pull/13901#pullrequestreview-2324294761:
```cpp
bool JsonExtensionType::ExtensionEquals(const ExtensionType& other) const {
  return other.extension_name() == this->extension_name();
}
```
> This equality check does not take into account the storage type, but only the name.
> As a consequence, a JsonExtensionType<string> type will be seen as equal to JsonExtensionType<large_string>.

### What changes are included in this PR?

This change introduces storage equality check into `JsonExtensionType` equality check.

This also fixes a storage type check in `JsonExtensionType::Make`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #44214

Lead-authored-by: Rok Mihevc <rok@mihevc.org>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: Antoine Pitrou <antoine@python.org>