Add Kotlin Ktor Hello world example (#3621)

Part of #3611

Working in the TodoMVC demo application. I'll raise a follow up PR for
that.

example/kotlinlib/web/1-hello-ktor/build.mill

@@ -0,0 +1,38 @@
+//// SNIPPET:BUILD
+package build
+import mill._, kotlinlib._
+
+object `package` extends RootModule with KotlinModule {
+
+  def kotlinVersion = "1.9.24"
+
+  def mainClass = Some("com.example.HelloKtorKt")
+
+  def ivyDeps = Agg(
+    ivy"io.ktor:ktor-server-core-jvm:2.3.12",
+    ivy"io.ktor:ktor-server-netty-jvm:2.3.12"
+  )
+
+  object test extends KotlinModuleTests with TestModule.Junit5 {
+    def ivyDeps = super.ivyDeps() ++ Agg(
+      ivy"io.kotest:kotest-runner-junit5-jvm:5.9.1", 
+      ivy"io.ktor:ktor-server-test-host-jvm:2.3.12"
+    )
+  }
+}
+
+// This example demonstrates how to set up a simple webserver 
+// serving a single "<h1>Hello, World!</h1>" web page using Kotlin and Ktor
+
+/** Usage
+
+> mill test
+
+> mill runBackground
+
+> curl http://localhost:8080
+...<h1>Hello, World!</h1>...
+
+> mill clean runBackground
+
+*/

example/kotlinlib/web/1-hello-ktor/src/com/example/HelloKtor.kt

@@ -0,0 +1,20 @@
+package com.example
+
+import io.ktor.server.application.*
+import io.ktor.server.engine.*
+import io.ktor.server.netty.*
+import io.ktor.server.response.*
+import io.ktor.server.routing.*
+
+fun main() {
+    embeddedServer(Netty, port = 8080, host = "0.0.0.0", module = Application::module)
+        .start(wait = true)
+}
+
+fun Application.module() {
+    routing {
+        get("/") {
+            call.respondText("<h1>Hello, World!</h1>")
+        }
+    }
+}

example/kotlinlib/web/1-hello-ktor/test/src/com/example/HelloKtor.kt

@@ -0,0 +1,22 @@
+package com.example
+
+import io.ktor.client.request.*
+import io.ktor.client.statement.*
+import io.ktor.http.*
+import io.ktor.server.testing.*
+
+import io.kotest.core.spec.style.FunSpec
+import io.kotest.matchers.shouldBe
+
+class HelloKtorTest: FunSpec({
+    test("HelloKtorTest") {
+        testApplication {
+            application {
+                module()
+            }
+        val response = client.get("/")
+        response.status shouldBe HttpStatusCode.OK
+        response.bodyAsText() shouldBe "<h1>Hello, World!</h1>"
+        }
+    }
+})

example/package.mill

@@ -43,6 +43,7 @@ object `package` extends RootModule with Module {
     object module extends Cross[ExampleCrossModuleKotlin](build.listIn(millSourcePath / "module"))
     object publishing extends Cross[ExampleCrossModuleKotlin](build.listIn(millSourcePath / "publishing"))
     object testing extends Cross[ExampleCrossModuleKotlin](build.listIn(millSourcePath / "testing"))
+    object web extends Cross[ExampleCrossModuleKotlin](build.listIn(millSourcePath / "web"))
   }
   object scalalib extends Module {
     object basic extends Cross[ExampleCrossModule](build.listIn(millSourcePath / "basic"))