[FLINK-23717] Allow setting configs using plain strings in StatefulFunctionsAppContainers

This closes #254.
diff --git a/statefun-e2e-tests/statefun-e2e-tests-common/src/main/java/org/apache/flink/statefun/e2e/common/StatefulFunctionsAppContainers.java b/statefun-e2e-tests/statefun-e2e-tests-common/src/main/java/org/apache/flink/statefun/e2e/common/StatefulFunctionsAppContainers.java
index 80bf7f1..863bcb6 100644
--- a/statefun-e2e-tests/statefun-e2e-tests-common/src/main/java/org/apache/flink/statefun/e2e/common/StatefulFunctionsAppContainers.java
+++ b/statefun-e2e-tests/statefun-e2e-tests-common/src/main/java/org/apache/flink/statefun/e2e/common/StatefulFunctionsAppContainers.java
@@ -239,6 +239,11 @@
       return this;
     }
 
+    public StatefulFunctionsAppContainers.Builder withConfiguration(String key, String value) {
+      this.dynamicProperties.setString(key, value);
+      return this;
+    }
+
     public StatefulFunctionsAppContainers.Builder withBuildContextFileFromClasspath(
         String buildContextPath, String resourcePath) {
       this.classpathBuildContextFiles.add(