OpenAPI emitter: allow operationId to be set on interfaces and namespaces to act as prefix specification

### Clear and concise description of the problem

```typespec
@OpenAPI.operationId("SomeOperations")
namespace Some {
op foo(): void;
}
```

This would result in foo having operationId = "SomeOperations_foo"

### Checklist

- [x] Follow our [Code of Conduct](https://github.com/microsoft/typespec/blob/main/CODE_OF_CONDUCT.md)
- [x] Read the [docs](https://typespec.io/docs/).
- [x] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.