What should we add?

We need to implement the box operation, as defined in Qiskit/RFCs#76.

We will also need to add support for custom annotations, and pass-manager handling for these, in a separate issue.

This issue is tracking just for the box operation itself. Related issues:

• Add convenience method to include an idle bit in control-flow builders #13767
• Add QASM3 stretch operation #13279