derive: provide a way to customize prost path.

[carllerche]

tower-grpc is based on Prost. I would like to improve the ergonomics of using it by not requiring an explicit dependency on Prost.

To do this, similar to #127, I will re-export prost from tower-grpc. The problem is that the generated derives require the prost crate to be added as a dependency.

One way to do this would be to:

• Allow the extern crate to be customized.
• Allow the path to prost to be customized.

This could be done with an extra attribute

#[derive(Message)]
#[prost(extern = "extern crate tower_grpc as _grpc", path = "_grpc::codegen::prost")]
pub struct PhoneNumber {
    // ...
}

[carllerche]

prost-build already supports the ability to specify custom type attributes, so tower-grpc's codegen could use that to inject that extra attribute.

[carllerche]

If this sounds good to you, I can work on a PR.