Native jni build (#5)

* Each native library is in its own jar. Got rid of cross compilation. The project is `secp256k1` and not `secp256k1-kmp`.

* Updated CI to use all 3 OS VMs

Co-authored-by: Salomon BRYS <salomon@kodein.net>

jni/jvm/linux/build.gradle.kts

@@ -0,0 +1,33 @@
+plugins {
+    kotlin("jvm")
+    `maven-publish`
+}
+
+dependencies {
+    implementation(project(":jni:jvm"))
+}
+
+val copyJni by tasks.creating(Sync::class) {
+    onlyIf { org.gradle.internal.os.OperatingSystem.current().isLinux }
+    dependsOn(":jni:jvm:buildNativeHost")
+    from(rootDir.resolve("jni/jvm/build/linux/libsecp256k1-jni.so"))
+    into(buildDir.resolve("jniResources/fr/acinq/secp256k1/jni/native/linux-x86_64"))
+}
+
+(tasks["processResources"] as ProcessResources).apply {
+    onlyIf { org.gradle.internal.os.OperatingSystem.current().isLinux }
+    dependsOn(copyJni)
+    from(buildDir.resolve("jniResources"))
+}
+
+publishing {
+    publications {
+        val pub = create<MavenPublication>("jvm") {
+            artifactId = "secp256k1-jni-jvm-linux"
+            from(components["java"])
+        }
+        if (!org.gradle.internal.os.OperatingSystem.current().isLinux) {
+            tasks.withType<AbstractPublishToMaven>().all { onlyIf { publication != pub } }
+        }
+    }
+}