extend qasm3 dumper with annotations in measurements

With this modification, qasm3 super extends annotations in measurements, which are only supported by mutable `Measure` subclass.

```python
from qiskit import qasm3
from qiskit.circuit import annotation, QuantumCircuit, Measure

class MidCircMeasurementAnnotation(annotation.Annotation):
    namespace = "qcf.mid_circ_measure"

class MidCircuitMeasure(Measure):
    """A custom specialized measurement."""

    def __init__(self, annotation_str=None):
        super().__init__(label=annotation_str)
        a = MidCircMeasurementAnnotation()
        a.namespace += "." + str(annotation_str)
        self.annotations = [a]
```

The annotation needs a serializers, but it is not used because, as defined above, does not have parameters:
```python
class Serializer(annotation.OpenQASM3Serializer):

    def dump(self, annotation):
        pass

    def load(self, namespace, payload):
        pass
```

It can be used in this way:
```python
measure_2 = MidCircuitMeasure("measure_2")
measure_3 = MidCircuitMeasure("measure_3")

circ = QuantumCircuit(2, 2)
circ.append(measure_2, [0], [0])
circ.append(measure_3, [1], [0])
circ.append(measure_2, [0], [1])
circ.measure_all()

handlers = {"qcf": Serializer()}
print(qasm3.dumps(circ, annotation_handlers=handlers))
```
```C
OPENQASM 3.0;
include "stdgates.inc";
bit[2] c;
bit[2] meas;
qubit[2] q;
@qcf.mid_circ_measure.measure_2AAA
c[0] = measure q[0];
@qcf.mid_circ_measure.measure_3
c[0] = measure q[1];
@qcf.mid_circ_measure.measure_2AAA
c[1] = measure q[0];
barrier q[0], q[1];
meas[0] = measure q[0];
meas[1] = measure q[1];
```

Author: 1ucian0

qiskit/qasm3/ast.py

@@ -388,11 +388,17 @@ class QuantumMeasurementAssignment(Statement):
         | indexIdentifier EQUALS quantumMeasurement  # eg: bits = measure qubits;
     """
 
-    __slots__ = ("identifier", "quantumMeasurement")
+    __slots__ = ("identifier", "quantumMeasurement", "annotations")
 
-    def __init__(self, identifier: Identifier, quantumMeasurement: QuantumMeasurement):
+    def __init__(
+        self,
+        identifier: Identifier,
+        quantumMeasurement: QuantumMeasurement,
+        annotations: Sequence[Annotation] = (),
+    ):
         self.identifier = identifier
         self.quantumMeasurement = quantumMeasurement
+        self.annotations = tuple(annotations)
 
 
 class Designator(ASTNode):

qiskit/qasm3/exporter.py

@@ -1003,7 +1003,22 @@ def build_current_scope(self) -> List[ast.Statement]:
                     [self._lookup_bit(operand) for operand in instruction.qubits]
                 )
                 clbit = self._lookup_bit(instruction.clbits[0])
-                nodes = [ast.QuantumMeasurementAssignment(clbit, measurement)]
+                nodes = [
+                    ast.QuantumMeasurementAssignment(
+                        clbit,
+                        measurement,
+                    )
+                ]
+                if hasattr(instruction.operation, "annotations"):
+                    annotations = [
+                        self.build_annotation(annotation)
+                        for annotation in instruction.operation.annotations
+                    ]
+                else:
+                    annotations = []
+                nodes = [
+                    ast.QuantumMeasurementAssignment(clbit, measurement, annotations=annotations)
+                ]
             elif isinstance(instruction.operation, Reset):
                 nodes = [
                     ast.QuantumReset(self._lookup_bit(operand)) for operand in instruction.qubits

qiskit/qasm3/printer.py

@@ -273,6 +273,8 @@ def _visit_QuantumMeasurement(self, node: ast.QuantumMeasurement) -> None:
         self._visit_sequence(node.identifierList, separator=", ")
 
     def _visit_QuantumMeasurementAssignment(self, node: ast.QuantumMeasurementAssignment) -> None:
+        for annotation in reversed(node.annotations):
+            self.visit(annotation)
         self._start_line()
         self.visit(node.identifier)
         self.stream.write(" = ")