How to customize ResourceKindProperty

Currently `ResourceKindProperty` is defined as:

https://github.com/Azure/typespec-azure/blob/433735c7fbb35a719ebde7e8d1cfa96303d36c58/packages/typespec-azure-resource-manager/lib/models.tsp#L282-L288

When it's mixed-in into a resource model:

```typespec
model Account is TrackedResource {
...ResourceKindProperty,
}
```
results in the following spec model:

```json
"kind": {
"type": "string",
"description": "Metadata used by portal/tooling/etc to render different UX experiences for resources of the same type; e.g. ApiApps are a kind of Microsoft.Web/sites type. If supported, the resource provider must validate and persist this value.",
"pattern": "^[-\\w\\._,\\(\\\\\\)]+$",
"x-ms-mutability": [
"read",
"create"
]
}
```

But I'd like to limit the value to an enum (list of well-known values), e.g. `direct` and `indirect`. Like what App Services have.

I could do it by defining my own model:

```typespec
union AccountKind {
Direct : "direct",
IndirectFoo : "indirect,foo",
IndirectBar: "indirect,bar",
string,
}
```

but it generates a warning:

>warning @azure-tools/typespec-azure-resource-manager/arm-resource-invalid-envelope-property: Property "kind" is not valid in the resource envelope. Please remove this property, or add it to the resource-specific property bag.

Start with ResourceKindProperty in packages/typespec-azure-resource-manager/lib/models.tsp at lines 282-288, then trace the ARM envelope validation that emits arm-resource-invalid-envelope-property. Define how a resource-specific kind union can replace or customize the standard property without triggering that warning, and verify that the generated schema preserves the allowed values.

Written by the indexing model from the issue text.