feat(build): allow optional service method return overrides

[NickLarsenNZ]

Motivation

As per my comments:

• Feature request: feat(build): Add optional default unimplemented stubs hyperium/tonic#1344 (comment)
• Motivation: Ability to provide default implementations on service traits tokio-rs/prost#844 (comment)

It would be nice to have configurable return values.

Solution

Warning:

• There are no tests (yet), and I have not run this, only run cargo build.
• We might need to consider adding proto package names to the mapping in case of duplicate service names in different packages.
• I have mostly copied existing patterns, so some signatures contain references even though copies/clones are occurring.
• Docs and variable/method names could use some massaging.

Here's an example of the feature in use (I have not yet tested it):

fn main() -> Result<(), Box<dyn std::error::Error>> {
   tonic_build::configure()
        .service_return_overrides(tonic_build::prost::ServiceReturnOverrideMap::from([
            (String::from("Echo.unary_echo"), ServiceReturnOverride::None), // implied if no mapping is made for the service. trait method implementation therefore required.
            (String::from("Echo.server_streaming_echo"), ServiceReturnOverride::OkDefault), // return the default value for the response message.
            (String::from("Echo.client_streaming_echo"), ServiceReturnOverride::ErrUnimplemented), // return the unimplemented error status.
            // any other service not mapped here will require an explicit trait method implementation.
        ]))
        .compile(
            &["proto/echo/echo.proto"],
            &["proto/echo"],
        )?;
   Ok(())
}

I did consider another variant to allow custom tonic::Status errors to be returned, but this was a little tricky without importing tonic into tonic-build - only for access to that single enum. It can be added later if the need surfaces.