New spread support

[qiaozha]

fixes #2414

In typespec, we have two kinds of spread.

1. the spread of model

model Foo {
   prop1: string;
   prop2: boolean;
}
op test(...Foo): void;

2. the spread of alias

alias Foo = {
   prop1: string;
   prop2: boolean;
};
op test(...Foo): void;

Previously, we have decided that we will treat spread of a alias as a payload flatten in this pr #2020 (comment) and spread of a model is not a payload flatten.

With payload flatten, the method signature would look like

export function test(prop1: string, prop2: boolean): void;

Without payload flatten, the method signature would be:

export function test(body: {prop1: string, prop2: boolean}): void;

And because alias was treated as anonymous model, we used to call getEffectivePayloadType from typespec compiler to try to get a valid payload type whenever possible, our previous spread condition becomes we flatten payload for anonymous model. it has a potential draw back which is we don't know if this model is defined as anonymous or become anonymous because of alias spread.

In Azure/typespec-azure#1043, we found that this kind of difference is too complex for customer to understand the client side behavior differently per on model or alias. we have decided to change that spread of a model is also a payload flatten.

In this PR, we have

1. integrate with the new spread behavior from tcgc, see [tcgc] new spread behavior typespec-azure#886 (comment) for more details.
2. mitigate the unnecessary flatten in our own test cases.

now spread is spread, anonymous model is anonymous model.

[joheredi]

Let's go with this fix to unblock moving the latest TCGC. In the meantime I'll setup some infrastructure so that we can move to using TCGC types in a more organized and smooth way