Add copyCertificates

- update README.md with certificate plugin
- add Ltd_hlaeja_plugin_hlaeja_common_plugin_certificate_gradle.kts

src/main/kotlin/ltd.hlaeja.plugin.hlaeja-common-plugin.certificate.gradle.kts

@@ -0,0 +1,18 @@
+val filesToCopy = listOf(
+    "private_key.pem",
+    "public_key.pem",
+    "keystore.p12",
+)
+
+tasks {
+    register<Copy>("copyCertificates") {
+        group = "hlaeja"
+        into("${layout.buildDirectory.get()}/resources/main/cert")
+        filesToCopy.filter { file("cert/$it").exists() }
+            .forEach { file ->
+                from("cert/$file") {
+                    include(file)
+                }
+            }
+    }
+}