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.

Checklist

• [X] Follow our Code of Conduct
• [X] Check that this issue is about the Azure libraries for typespec. For feature request in the typespec language or core libraries file it in the TypeSpec repo
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[markcowl]

@markcowl schedule for dpg discussion