Add support for const sweep with None (#6729)

* Add support for const sweep with None

* Fix the test

* Fix the test coverage

* Fix the test coverage

* Address the comment

* Use const for all single sweep

* Add more test for constant

* Fix typecheck

* Fix the lint

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

@@ -179,13 +179,16 @@ message SingleSweep {
     Points points = 2;
     // Uniformly-spaced sampling over a range.
     Linspace linspace = 3;
+    // A constant value.
+    Const const = 5;
   }
 
   // Optional arguments for if this is a device parameter.
   // (as opposed to a circuit symbol)
   DeviceParameter parameter = 4;
 }
 
+
 // A list of explicit values.
 message Points {
   // The values.
@@ -207,3 +210,15 @@ message Linspace {
   // the same.
   int64 num_points = 3;
 }
+
+// A constant value.
+message Const {
+  // The values.
+  oneof value {
+    // This value should always be true if set, which represent the python None object.
+    bool is_none = 1;
+    float float_value = 2;
+    int64 int_value = 3;
+    string string_value = 4;
+  }
+}