Dynamically named services

## Feature Request

Currently the `NamedService` trait assumes that codegen from Prost is used to generate at compile time a const field that has a service name. This trait must be implemented on a service in order for the router to accept it (`add_service` in `router.rs` requires the `NamedService` trait).

This prevents the creation of services with names not known at compile time.

### Motivation

I am creating an IDL agnostic server, using FFI to bridge to C++ which will handle the payload as the server is unable to serialize or deserialize it. In this scheme, the C++ client code will register methods onto a service with callbacks. Since the server will not know at compile time what the services it will be running are called, I currently must use a single placeholder service name.

### Proposal

```
pub trait NamedService {
const NAME: &'static str;
}
```

Needs to shift to something more like:

```
pub trait NamedService {
fn get_name(self) -> &str;
}
```

And the subsequent internal modifications to both Tonic and Prost to support this need implemented.

I am new to Rust and not confident enough to implement this publicly. Suggestions and feedback welcomed.