openapi/json-schema - `@extension` alignment - emit schemas

This issue tracks updating the `@typespec/openapi` `@extension` decorator to emit passed in Types as Open API schemas, similar to how `@typespec/json-schema` will emit JSON Schemas for passed in types.

For example:
```tsp
@OpenAPI.extension("x-foo", { foo: true })
model Foo {}
```
will generate a schema since `{ foo: true }` is a Type.

```yaml
Foo:
type: object
x-foo:
type: object
required:
- foo
properties:
foo:
type: boolean
enum:
- true
```

This can be considered a breaking change for anyone passing in a model or tuple expression into `@OpenAPI.extension`. To migrate, users will need to use Value kinds instead.

For example, `{ foo: true }` would be updated to `#{ foo: true }`.

This should not be completed until after #6076 has been included in a release.