feat(build): Support prost's include_file option

[DerekStrickland]

This PR adds pass-through calls to post to allow tonic users to make use of a new function in prost called include_file.

Motivation

The include_file function provides users with a way to more elegantly handle compiling a set of .proto files with complex dependency hierarchies. Traditionally, users would either need to emulate the folder structure of their .proto sources or manually configure a mod.rs that accurately mapped the source .proto folder structure to a module graph. Now, include_file will do that for you.

Solution

Assuming some complex .proto source folder structure and dependency graph, users can add a call to include_file in their configuration like this.

tonic_build::configure()
        .include_file("mod.rs")
        .compile(
            &[
                "somesource/plugins/base/proto/base.proto",
                "somesource/plugins/drivers/proto/driver.proto",
                "somesource/plugins/shared/foo/foo.proto",
                "somesource/plugins/shared/structs/proto/attribute.proto",
            ],
            &["somesource"],
        )
        .unwrap();

By configuring include_file in the config as shown, prost will now generate a mod.rs that looks like this.

pub mod somesource {
      pub mod plugins {
          pub mod shared {
              pub mod foo {
                  include!("somesource.plugins.shared.foo.rs");
              }
              pub mod structs {
                  include!("somesource.plugins.shared.structs.rs");
              }
          }
          pub mod base {
              pub mod proto {
                  include!("somesource.plugins.base.proto.rs");
              }
          }
          pub mod drivers {
              pub mod proto {
                  include!("somesource.plugins.drivers.proto.rs");
              }
          }
      }
}

[tobz]

Tested this locally, with and without .include_file(..) and the results look valid, and most importantly, .include_file(..) in fact does the right thing. :)

[LucioFranco]

Thanks!