Location of DataFactoryExpression to support Data Factory Expression

### Clear and concise description of the problem

To be able to support [dfe-string](https://github.com/Azure/azure-rest-api-specs/blob/c4e661cdf92c8f579574008d0cd11874cc303da0/specification/datafactory/resource-manager/Microsoft.DataFactory/stable/2018-06-01/entityTypes/Pipeline.json#L7580), [dfe-int](https://github.com/Azure/azure-rest-api-specs/blob/c4e661cdf92c8f579574008d0cd11874cc303da0/specification/datafactory/resource-manager/Microsoft.DataFactory/stable/2018-06-01/entityTypes/Pipeline.json#L7671), [dfe-key-value-pairs](https://github.com/Azure/azure-rest-api-specs/blob/c4e661cdf92c8f579574008d0cd11874cc303da0/specification/datafactory/resource-manager/Microsoft.DataFactory/stable/2018-06-01/entityTypes/Pipeline.json#L7600) and [dfe-list-generic](https://github.com/Azure/azure-rest-api-specs/blob/main/specification/datafactory/resource-manager/Microsoft.DataFactory/stable/2018-06-01/entityTypes/Dataset.json#L35-L36), 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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