[Feature Request] Need some way to declare the protocol(s) of a service

### Clear and concise description of the problem

Currently, the HTTP library considers every namespace decorated with `@service` to be an HTTP service, but that is not necessarily the case. `@service` is also used for Protobuf/gRPC, so we should have some way to declare the protocol of a service so that the HTTP emitter will ignore it if the protocol is set to something else.

This is important for multi-protocol specs to work well.

### 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.