Provide a protoc-gen-kotlin maven artifact that's compatible with the protobuf gradle plugin

[garyp]

The protobuf gradle plugin supports downloading protoc plugins from maven central: https://github.com/google/protobuf-gradle-plugin#locate-external-executables. This would make it much easier to use pb-and-k in a gradle build since the protoc-gen-kotlin binary wouldn't have to be compiled/installed manually.

The requirements to enable this are:

• Package protoc-gen-kotlin as a self-executable jar. For example, you can use something like the Spring Boot Gradle Plugin to create a fully executable archive.

• Distribute the self-executable jar on maven central with a custom classifier (the protobuf gradle plugin doesn't provide any way to use an artifact with an empty classifier).

For example, if the self-executable jar is distributed as the com.github.cretz.pbandk:protoc-gen-kotlin artifact with a jvm8 classifier, then it could be used with the protobuf gradle plugin like this:

 protobuf {
    plugins {
        kotlin {
            artifact = 'com.github.cretz.pbandk:protoc-gen-kotlin:0.3.0:jvm8@jar'
        }
    }
    ...
}