feat: Kotlin SDK

[BulkBeing]

Provide idiomatic Kotlin APIs around existing Java SDK.

• This PR only targets implementing the Sinker APIs and publishing the package.
• Kotlin and Java sdk will follow different versioning (at least initially). There will be separate pipeline to publish kotlin sdk.

A sink implementation in kotlin using this sdk would look like:

import io.numaproj.numaflow.kt.sinker.*

fun main() {
    sinkServer { datums ->
        datums.processEach { datum ->
            when {
                datum.keys.isEmpty() -> datum.fallback()
                datum.value.isEmpty() -> datum.failure("empty payload")
                else -> {
                    println("Received: $datum")
                    datum.ok()
                }
            }
        }
    }.run()
}

The same would look like this if the user uses existing Java sdk in kotlin:

fun main() {
     val sinker = object : Sinker() {
         override fun processMessages(datumStream: DatumIterator): ResponseList {
             val builder = ResponseList.newBuilder()
             while (true) {
                 val datum = datumStream.next() ?: break
                 val response = when {
                     datum.keys.isNullOrEmpty() -> Response.responseFallback(datum.id)
                     datum.value == null || datum.value.isEmpty() -> Response.responseFailure(datum.id, "empty payload")
                     else -> {
                         println("Received: $datum")
                         Response.responseOK(datum.id)
                     }
                 }
                 builder.addResponse(response)
             }
             return builder.build()
         }
     }

     Server(sinker).apply {
         start()
         awaitTermination()
     }
}

NOTE: My only experience with Kotlin is reading the book "Kotlin in Action, Second Edition". So I'm expecting the sdk APIs may change a lot as I gain more experience using Kotlin and getting feedback from Kotlin devs.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

@@           Coverage Diff           @@
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[KeranYang]

@BulkBeing Thanks for doing this. Why don't we create a numaflow-kotlin repository? To me, it's cleaner with single responsibility. Having a numaflow-kotlin folder under numaflow-java repository doesn't look right to me.

[BulkBeing]

@BulkBeing Thanks for doing this. Why don't we create a numaflow-kotlin repository? To me, it's cleaner with single responsibility. Having a numaflow-kotlin folder under numaflow-java repository doesn't look right to me.

I've seen some repos that provide kotlin apis with core implemented in Java having both files in same repo. In our case, I think separate repo makes sense, since we need to manage different versions (at-least initially) as Kotlin codebase will change more. Also, we need replicate all examples as well.

[vigith]

Let's work towards the target state. If the target state is single repo, let's just stick to single repo.

How will the artifact be published? Independently (Java and Kotlin together) or together?

[vigith]

nm.. I see that the artifact is separate.