Introduce warning silencer plugin (#26588)

So now we can compile akka-distributed-data with
-Xfatal-warnings - though I'm not yet sure about
enabling the (other) undisciplineScalacOptions

* Fix multi-node silencing
* Fix scaladoc warnings
* Introduce annotation to declare ccompat use
* Add explicit toString
* Fix deprecation on 2.13
* Move 'immutable' ccompat helpers to shared ccompat package
* Add MiMa for internal scala 2.13 compatibility class
* Internal API markers
* Fix scaladoc generation
Got bitten by https://github.com/scala/bug/issues/11021

project/MultiNode.scala

@@ -68,6 +68,8 @@ object MultiNode extends AutoPlugin {
     SbtMultiJvm.multiJvmSettings ++
       inConfig(MultiJvm)(scalafmtConfigSettings) ++
       Seq(
+        // Hack because 'provided' dependencies by default are not picked up by the multi-jvm plugin:
+        managedClasspath in MultiJvm ++= (managedClasspath in Compile).value.filter(_.data.name.contains("silencer-lib")),
         jvmOptions in MultiJvm := defaultMultiJvmOptions,
         scalacOptions in MultiJvm := (scalacOptions in Test).value,
         logLevel in multiJvmCreateLogger := Level.Debug, //  to see ssh establishment