add custom attribute for generated service

[luckyuro]

I'd like to use Box for the twurst_build generated Service.
It will require to use the #[async_trait] for serivce from async_trait.

twurst_build::TwirpBuilder::new()
        .with_server()
        .with_client()
        .with_custom_serivce_attribute("#[::async_trait::async_trait]")
        .compile_protos(&[...], &[...])

the generated code will suppose to be

#[::async_trait::async_trait]
pub trait TwirpService {
...
}

I'm not sure whether this would be a common case, and could merge to main branch.
So I'd just put the PR and maybe someone could check.

[vsiles]

@Tpt We might want to have something like tonic_prost_build.with_type_attribute for twurst ;)

I'm on a trip at the moment, I'll get a look probably next week

[Tpt]

In the meantime I think you can impl MyService for Box<dyn MyServiceImplementation> where MyServiceImplementation is your implementation of MyService. It's a bit of boilerplate but it should work.