[Dpg][Bug] Support type narrow down

[ArcturusZhang]

Describe the issue or request

For instance we could have this in tsp:

model Foo {
    p: int64;
}

model Bar extends Foo {
    p: int32;
}

in which the model Bar extends Foo and narrows down the type of property p inherited from Foo.

We currently are not supporting this properly (the generator throws key already added exception).

[ArcturusZhang]

Narrow down could also happen with union types, such as:

model Foo {
    p: string | int32;
}

model Bar extends Foo {
    p: string;
}

or with literal unions

model Foo {
    p: "a" | "b";
}

model Bar extends Foo {
    p: "b";
}

[ArcturusZhang]

Even this could compile as a tsp:

model Foo {
  p: string;
}

model Bar extends Foo {
  p?: string;
}

or vice versa

model Foo {
  p?: string;
}

model Bar extends Foo {
  p: string;
}