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/all/build.gradle.kts

@@ -0,0 +1,20 @@
+plugins {
+    `java-library`
+//    `maven-publish`
+    id("ru.vyarus.pom") version "2.1.0"
+}
+
+dependencies {
+    api(project(":jni:jvm:darwin"))
+    api(project(":jni:jvm:linux"))
+    api(project(":jni:jvm:mingw"))
+}
+
+publishing {
+    publications {
+        create<MavenPublication>("jvm") {
+            artifactId = "secp256k1-jni-jvm"
+            from(components["java"])
+        }
+    }
+}