Make NestedObjectMapper its own class (elastic#74410)

Nested objects are implemented via a Nested class directly on object mappers,
even though nested and non-nested objects have quite different semantics. In
addition, most call-sites that need to get an object mapper in fact need a nested
object mapper. To make it clearer that nested and object mappers are different
beasts with different implementations and different requirements, we should
split them into different classes.

server:test compiles

server/src/main/java/org/elasticsearch/index/mapper/RootObjectMapper.java

@@ -130,7 +130,6 @@ private static void fixRedundantIncludes(ObjectMapper objectMapper, boolean pare
                     child.setIncludeInParent(true);
                     child.setIncludeInRoot(false);
                 }
-                fixRedundantIncludes(child, includeInRootViaParent || includedInRoot);
             }
         }
     }