[protobuf] Support setting fileName in TypeSpec.Protobuf.PackageDetails

[kolaworld]

Clear and concise description of the problem

The TypeSpec.Protobuf.package decorator name property results in a generated file rather than a directory matching the package name.

Current State

Given the following:

import "@typespec/protobuf";

using TypeSpec.Protobuf;

@package({
    name: "my.example.v1",
    options: {
        go_package: "github.com/org/repo/go/my/example/v1",
    },
})
namespace Example;

The protobuf emitter emits a file /my/example/v1.proto, see playground.

I would expect /my/example/v1/generated.proto

Problem

This prevents the ability to add multiple manual .proto files along side the generated proto under the same package "my.example.v1".

• Given /my/example/v1.proto, I cannot add /my/example/v1/non-generated.proto

This also breaks the linter for the popular bufbuild tool which expects the package name to match a directory path minus a filename - "my.example.v1" -> /my/example/v1/.

Solution (non-breaking)

Add a property fileName to the TypeSpec.Protobuf.PackageDetails to allow generating proto files with a specific filename.

If no fileName is provided, maintain backwards compatibility by emitting the current state.

Example with fileName would result in emitted file: /my/example/v1/cool.proto

@package({
    name: "my.example.v1",
    fileName: "cool"
    options: {
        go_package: "github.com/org/repo/go/my/example/v1",
    },
})
namespace Example;

Other considerations

I imagine it is better to add a new property fileName to TypeSpec.Protobuf.PackageDetails rather than a new protobuf emitter --option="@typespec/protobuf.file-name=xxx" as the former would persist the file name to support .proto import directives.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.