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GH-43453: [Format] Add Opaque canonical extension type (#43457)

### Rationale for this change

Add the newly ratified extension type.

### What changes are included in this PR?

The type specification only.

### Are these changes tested?

N/A

### Are there any user-facing changes?

No.
* GitHub Issue: #43453

Lead-authored-by: David Li <li.davidm96@gmail.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: David Li <li.davidm96@gmail.com>