Add Gradle Plugin Portal publication to the buildscripts

Add option to disable signing (-PnoSign), to be used when publishing
pre-built artifacts

Add option to define specific version for publishing, to be used when
publishing test versions

(cherry picked from commit dcd55e9)

libraries/tools/kotlin-allopen/build.gradle

@@ -33,4 +33,17 @@ compileKotlin.dependsOn preprocessSources
 
 jar {
     from(targetSrc) { include("META-INF/**") }
+}
+
+pluginBundle {
+    plugins {
+        kotlinAllopenPlugin {
+            id = 'org.jetbrains.kotlin.plugin.allopen'
+            description = displayName = 'Kotlin All Open compiler plugin'
+        }
+        kotlinSpringPlugin {
+            id = 'org.jetbrains.kotlin.plugin.spring'
+            description = displayName = 'Kotlin Spring compiler plugin'
+        }
+    }
 }