Publish plugin artifact for external plugin usage

[buildbreaker]

Currently, the plugin is only published to the BSR. It would be nice for users of Connect-Kotlin to also be able to consume the plugin independently from the BSR. The options for this are:

1. Publish directly to maven as another artifact: build.buf:connect-kotlin:protoc-gen-connect-kotlin
2. Add an additional artifact within the releases which is the plugin jar

[kohenkatz]

For users who are already using grpc-kotlin or grpc-java, I think that publishing an artifact to Maven is the easiest way to get them on board. The reason for this is that anyone who is already using the protobuf Gradle plugin can add connect-kotlin with just a few lines in the build.gradle{.kts} file.

Here's a snippet from build.gradle.kts that uses the protobuf plugin:

protobuf {
    protoc {
        artifact = "com.google.protobuf:protoc:${rootProject.ext["protobufVersion"]}"
    }
    plugins {
        id("java") {
            artifact = "io.grpc:protoc-gen-grpc-java:${rootProject.ext["grpcVersion"]}"
        }
        id("grpc") {
            artifact = "io.grpc:protoc-gen-grpc-java:${rootProject.ext["grpcVersion"]}"
        }
        id("grpckt") {
            artifact = "io.grpc:protoc-gen-grpc-kotlin:${rootProject.ext["grpcKotlinVersion"]}:jdk8@jar"
        }
    }
    generateProtoTasks {
        all().forEach {
            it.plugins {
                id("java") {
                    option("lite")
                }
                id("grpc") {
                    option("lite")
                }
                id("grpckt") {
                    option("lite")
                }
            }
            it.builtins {
                id("kotlin") {
                    option("lite")
                }
            }
        }
    }
}

If the generator is published in maven central, something like this is probably all that's needed to make it work:

        id("connectkt") {
            artifact = "build.buf:protoc-gen-connect-kotlin:${rootProject.ext["connectKotlinVersion"]}:jdk8@jar"
        }

and

            it.plugins {
                // ...
                id("connecetkt") {
                }
            }

However, for users who want to get away from protoc and use buf command line, an artifact in GitHub releases is probably much easier to download.

[kohenkatz]

I tested my comment above by building the JAR and running it like this:

        id("connectkt") {
            path = "/path/to/project/protoc-gen-connect-kotlin.jar"
        }

As far as I can tell so far, this works perfectly (except for the issue identified in #6).

[buildbreaker]

This should be available in the next release (v0.1.3)