refactor!: replace Closeable by explicit Stoppable interface

This ensures that stoppable classes can also be started if needed.
Fixes #629

Author: metacosm

operator-framework-core/src/main/java/io/javaoperatorsdk/operator/api/Stoppable.java

@@ -0,0 +1,9 @@
+package io.javaoperatorsdk.operator.api;
+
+import io.javaoperatorsdk.operator.OperatorException;
+
+public interface Stoppable {
+  void start() throws OperatorException;
+
+  void stop() throws OperatorException;
+}