Add Serialization and Deserialization Support of FSimViaModelTag. (quantumlib#6539)

* Add Serialization and Deserialization Support of FSimViaModelTag.

* Fix the lint

* Update program_pb2.pyi

Author: BichengYing

Diff for: cirq-google/cirq_google/api/v2/program.proto

@@ -167,6 +167,11 @@ message CZPowGate {
 message FSimGate {
   FloatArg theta = 1;
   FloatArg phi = 2;
+
+  // If true, this is equivalent to:
+  // cirq.FSimGate(...).with_tags(cirq_google.FSimViaModelTag()).
+  // This field controls how we translate the gate implementation.
+  bool translate_via_model = 3;
 }
 
 // Representation of cirq.ISwapPowGate