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GH-43454: [C++][Python] Add Opaque canonical extension type (#43458)

### Rationale for this change

Add the newly ratified extension type.

### What changes are included in this PR?

The C++/Python implementation only.

### Are these changes tested?

Yes

### Are there any user-facing changes?

No.
* GitHub Issue: #43454

Lead-authored-by: David Li <li.davidm96@gmail.com>
Co-authored-by: Weston Pace <weston.pace@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>