tonic_build clone server

Add the option to generate Server wrappers that clones the inner type instead of using `Arc`.

```rust
/// Service generator builder.
#[derive(Debug, Clone)]
pub struct Builder {
...
// These should be mutually exclusive, possibly an Enum
pub(crate) use_arc_self: bool,
pub(crate) use_clone_self: bool,
...
}
```

If this seems reasonable, I am more than happy to implement it my self.