Implement get and containsKey in terms of the wrapped innerMap (elastic#77965)

Author: joegallo

server/src/main/java/org/elasticsearch/cluster/metadata/DiffableStringMap.java

@@ -47,6 +47,16 @@ public Set<Entry<String, String>> entrySet() {
         return innerMap.entrySet();
     }
 
+    @Override
+    public String get(Object key) {
+        return innerMap.get(key);
+    }
+
+    @Override
+    public boolean containsKey(Object key) {
+        return innerMap.containsKey(key);
+    }
+
     @Override
     @SuppressWarnings("unchecked")
     public void writeTo(StreamOutput out) throws IOException {