code: public SchemaBuilder mapper(String field, MapperBuilder<?, ?> mapper) {
          mapperBuilders.put(field, mapper);
          return this;
      }
docs: Adds a new {@link Mapper}.
  
  @param field the name of the {@link Mapper} to be added
  @param mapper the builder of the {@link Mapper} to be added
  @return this with the specified mapper