update publication scripts

tgbotapi.extensions.api/publish.gradle

@@ -1,8 +1,12 @@
 apply plugin: 'maven-publish'
+apply plugin: 'signing'
 
 task javadocsJar(type: Jar) {
     classifier = 'javadoc'
 }
+task sourceJar (type : Jar) {
+    classifier = 'sources'
+}
 
 afterEvaluate {
     project.publishing.publications.all {
@@ -10,6 +14,7 @@ afterEvaluate {
         groupId "${project.group}"
         if (it.name.contains('kotlinMultiplatform')) {
             artifactId = "${project.name}"
+            artifact sourceJar
         } else {
             artifactId = "${project.name}-$name"
         }
@@ -62,4 +67,9 @@ publishing {
         }
         
     }
-}
+}
+
+signing {
+    useGpgCmd()
+    publishing.publications.forEach { sign it }
+}