ADR: Composite Inline VariableMutations

[coltmcnealy-lh]

It is annoying that we can't do something like:

wf.mutate(myVar, ASSIGN, someOtherVar.multiply(10));

It would be nice to have this code do the same as myVar = someOtherVar * 10, without mutating someOtherVar. Right now, we have to define a separate internal variable to do this.

message VariableAssignment {
  message FormatString {
    VariableAssignment format = 1;
    repeated VariableAssignment args = 2;
  }
  optional string json_path = 1;

  // THIS IS NEW
  message CompoundAssignment {
    VariableAssignment lhs = 1;
    VariableMutationType mutation = 2;
    VariableAssignment rhs = 3;
  }

  oneof source {
    string variable_name = 2;
    VariableValue literal_value = 3;
    FormatString format_string = 4;

    // THIS IS NEW
    CompoundAssignment compound = 5;
  }
}

This would enable something like the following in our SDK:

// wf.execute("some-task", myIntVariable.times(10));