add support for any type of cmd schema in typespec-aaz

[AllyW]

tsp model 1:

@doc("The output value.")
 value?: unknown;

output:

{
    "type": "any",
    "name": "value",
    "description": "The output value."
}

tsp model 2:

 @doc("Custom operation properties, populated only for a successful operation.")
  properties?: Record<unknown>;

output:

{
    "type": "object",
    "additionalProps": {
        "item": {
            "type": "any"
        }
    },
    "name": "properties",
    "description": "Custom operation properties, populated only for a successful operation."
}