Add missing setter for connection in MongoDb4Provider builder (#1390)

Author: ihordz

log4j-mongodb4/src/main/java/org/apache/logging/log4j/mongodb4/MongoDb4Provider.java

@@ -59,6 +59,11 @@ public MongoDb4Provider build() {
             return new MongoDb4Provider(connection, capped, collectionSize);
         }
 
+        public B setConnectionStringSource(final String connection) {
+            this.connection = connection;
+            return asBuilder();
+        }
+
         public B setCapped(final boolean isCapped) {
             this.capped = isCapped;
             return asBuilder();