feat(sidecar): scaffold Kotlin JVM module with Shadow fat JAR

Kotlin 2.1.21 (deviation from plan's 1.9, forced by Gradle 9
compatibility), JDK 17 toolchain, application plugin with main
entry dev.uatu.sidecar.MainKt. Shadow 9.0.0-rc2 for the fat JAR.
Actual gRPC server wires in with task #10.

sidecar/build.gradle.kts

@@ -0,0 +1,28 @@
+plugins {
+    kotlin("jvm") version "2.1.21"
+    application
+    id("com.gradleup.shadow") version "9.0.0-rc2"
+}
+
+java {
+    toolchain {
+        languageVersion.set(JavaLanguageVersion.of(17))
+    }
+}
+
+kotlin {
+    jvmToolchain(17)
+}
+
+dependencies {
+    testImplementation(kotlin("test"))
+    testImplementation("org.junit.jupiter:junit-jupiter:5.11.3")
+}
+
+application {
+    mainClass.set("dev.uatu.sidecar.MainKt")
+}
+
+tasks.test {
+    useJUnitPlatform()
+}