Preserve original input type information for external types

[JoshLove-msft]

For the case of DFE in particular, we expect tsp like the following:

@doc("Expression model for DataFactory expressions")
model DfeExpressionModel {
  @doc("The type of expression")
  type: "Expression";
  @doc("The expression value")
  value: string;
}

// DataFactoryElement alternate type pattern
@alternateType({identity: "Azure.Core.Expressions.DataFactoryElement"}, "csharp")
union Dfe<T> {
  T,
  DfeExpressionModel
}

@doc("Model with DataFactoryElement properties")
model DataFactoryElementModel {
  @doc("String property with DFE pattern")
  stringProperty: Dfe<string>;

  @doc("Int property with DFE pattern")
  intProperty: Dfe<int32>;

  @doc("Bool property with DFE pattern")
  boolProperty: Dfe<boolean>;

  @doc("String array property with DFE pattern")
  stringArrayProperty: Dfe<string[]>;
}

Right now, the union is replaced in the C# emitter with an external type. We should preserve the original type and just include the external info as metadata on InputType the same way that TCGC does. This is needed for DFE as we need to be able to get the union variant that represents the generic argument. There may be similar scenarios for other alternate types so we should just preserve the original type.