Fix casting issue

In IntelliJ, the existing combination of raw types, casting, and lambdas often produces spurious compilation errors. This PR introduces a solution which hides the unsafe cast in a single method, and also uses `Map.putIfAbsent` to avoid constructing a lambda for this case.

implementation/src/main/java/io/smallrye/config/SingleConfigSourceInterceptor.java

@@ -0,0 +1,4 @@
+package io.smallrye.config;/**
+ * 
+ */public final class SingleConfigSourceInterceptor {
+}

implementation/src/main/java/io/smallrye/config/util/Iterators.java

@@ -0,0 +1,4 @@
+package io.smallrye.config.util;/**
+ * 
+ */public final class Iterators {
+}