Need a way to disable arg marshalling for decorators

### Clear and concise description of the problem

There is some decorators what would want to get the original value non marshalled(like the `@example` decorator).

There is a few options:
1. add a `getRawArg(index: number): Entity` on the `DecoratorContext`. - Problem is you cannot call then a decorator with `context.call()` that depend on this.
2. Add a `$example.disableArgMarshalling` - Problem is that `tspd` then cannot generate the right signature for this decorator as it won't know about this flag.
3. Add a modifier on the decorator declaration `extern raw dec example(target, value: valueof unknown)`

### 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.