Add Quantum Engine Support for InternalGate (#6613)

* add internal gate to device proto

* build protos

* tests pass

* add docstring

* build protos

* build protos

Author: senecameeks

cirq-google/cirq_google/api/v2/device.proto

@@ -58,6 +58,7 @@ message GateSpecification {
     Wait wait = 11;
     FSimViaModel fsim_via_model = 12;
     CZPowGate cz_pow_gate = 13;
+    InternalGate internal_gate = 14;
   }
 
   // Gate types available to Google devices.
@@ -76,6 +77,9 @@ message GateSpecification {
   message Wait {}
   message FSimViaModel {}
   message CZPowGate {}
+  // This gate gets mapped to the internal representation corresponding
+  // to <gate_module.gate_name>.
+  message InternalGate {}
 }
 
 message GateSet {

cirq-google/cirq_google/api/v2/device_pb2.py

@@ -164,6 +164,9 @@ class _GateRepresentations:
         gate_spec_name='fsim_via_model',
         supported_gates=[cirq.GateFamily(cirq.FSimGate, tags_to_accept=[ops.FSimViaModelTag()])],
     ),
+    _GateRepresentations(
+        gate_spec_name='internal_gate', supported_gates=[cirq.GateFamily(ops.InternalGate)]
+    ),
 ]