Add DAG substitute op to the C API.

[kevinhartman]

What should we add?

We need a qk_dag_substitute_op function(s) so users can replace an operation within the DAG.

This might be more involved, depending on what's needed. We don't currently have a way to create a standalone QkCircuitInstruction to use as the replacement, and we don't have a QkOperation, to my knowledge.

We might need to have a few specialized functions for specific operation types:

• qk_dag_substitute_op_with_gate
• qk_dag_substitute_op_with_unitary

[kevinhartman]

Downgraded to P1, since we can technically use qk_dag_substitute_node_with_dag to do this (and that one is definitely P0).

[Cryoris]

Moving to 2.4 as there's no associated PR by now and there's an existing alternative path.

[alexanderivrii]

Hi @kt474, we are two weeks away from the 2.4 feature freeze, let me know if this is still something you are planning to do, thanks!