Ability to configure server and client mod names

## Feature Request

### Crates

- `tonic-build`

### Motivation

I'd like to reduce stutter in my pb definitions. For example if I have a service `example` (`service Example`)
```rust
pub mod example {
include!(concat!(env!("OUT_DIR"), "/example.rs"));
}
```

I'd like to be able to use it like this

```rust
use pb::example::server::ExampleServer;
```

### Proposal

Allow a way to configure the name of generated the server and client modules.

The relevant code is the following

```rust
let server_mod = quote::format_ident!("{}_server", naive_snake_case(service.name()));
```

And
```rust
let client_mod = quote::format_ident!("{}_client", naive_snake_case(service.name()));
```