添加 DAGCircuit.get_var() (以及 get_stretch())，以与 QuantumCircuit 和 CircuitData 保持一致

Expected behavior

Add Python methods on DAGCircuit, mirroring CircuitData / QuantumCircuit:

dag.get_var(name: str) -> expr.Var | None
dag.get_stretch(name: str) -> expr.Stretch | None

Optionally also get_identifier(name) for symmetry with has_identifier.

Suggested semantics (matching CircuitData):

• Return the expr.Var / expr.Stretch for the given name, or None if not found.
• Alternatively, support a default kwarg like QuantumCircuit.get_var for API consistency across the stack.