Location of DataFactoryExpression to support Data Factory Expression

[live1206]

Clear and concise description of the problem

To be able to support dfe-string, dfe-int, dfe-key-value-pairs and dfe-list-generic, we decided to have a common type like below

model DataFactoryExpression {
	type: "Expression";
	value: string;
}

model MyModel {
  myProp : int32 | DataFactoryExpression
}

We need to finalize the location of the common model DataFactoryExpression.

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[markcowl]

@markcowl schedule for dpg discussion