[TCGC] client type for TypeSpec empty model

[tadelesh]

In modeler4, we have anyObject type to represent an empty model definition despite whether it is a named definition or not (see the type of prop1 and prop2 for the following example). And most languages I believe will treat it as any type.

"definitions": {
  "EmptyModel": {
    "type": "object"
  },
  "Test": {
    "type": "object",
    "properties": {
      "prop1": {
        "type": "object"
      },
      "prop2": {
        "$ref": "#/definitions/EmptyModel"
      }
    },
    "required": [
      "prop1",
      "prop2"
    ]
  }
}

But in TypeSpec, we could clearly say a model is a named model without any properties or an anonymous model without any properties (type of prop1 and prop2).

model Test {
  prop1: {};
  prop2: EmptyModel;
}
model EmptyModel {}

So, we need to get consensus on what type we should generate for TypeSpec empty model, and also it should support non-breaking change for brown field services.

Proposal:
Introduce anyObject type in TCGC. Anonymous model without any properties will be converted to anyObject. For now, unknown is converted to any in TCGC.
Named model without any properties will be kept as a named model.
For brown field service, converter should always use anonymous model in TypeSpec without any properties to represent anyObject type in m4.

cc: @Azure/dpg-devs

[tadelesh]

Conclude all possible cases in the following table, example is in this playground:

Swagger	m4	TypeSpec	TCGC	example
{"type": "object"}	anyObject	{}	anonymous model with no properties	prop1
{"type": "object"}	anyObject	model EmptyModel {}	named model with no properties	prop2
{}	any	unknown	any	prop3
{"type": "object", "additionalProperties": {}}	dictionary with any type	Record<unknown>	dictionary with any type	prop4
{"type": "object", "additionalProperties": {}}	dictionary with any type	model EmptyModelWithAdditionalProperties extends Record<unknown> {}	named model with only additional properties	prop5
{"type": "object", "additionalProperties": {}}	dictionary with any type	model EmptyModelWithAdditionalProperties is Record<unknown> {}	named model with only additional properties	prop6
{"type": "object", "additionalProperties": {}}	dictionary with any type	model EmptyModelWithAdditionalProperties {...Record<unknown>}	named model with only additional properties	prop7

@timotheeguerin could you help to confirm this is the right behavior for swagger/m4?
@Azure/dpg-devs: please confirm the behavior for TCGC.

Since swagger/m4 one expression could map several TypeSpec expression, for brown field service converting to TypeSpec, we will use the following mapping:

m4	TypeSpec
anyObject	{}
any	unknown
dictionary with any type	Record<unknown>

[iscai-msft]

Agree with the mapping at the end, we shouldn't do any extra conversion bc of the possibility of breaking changes

[pshao25]

Decision: everything keeps the same, only in conversion tool, change anyObject in M4 to {} in TypeSpec.

[tadelesh]

since typespec has more expression ability than swagger, we decided to use more accurate result when converting from swagger to typespec. if the conversion result is not the one service want, typespec author needs to change manually.
regarding how languges' emitter deal with the empty model is depends on languages' architect. tcgc will keep the full info and do no implicit conversion.