FeatureRequest: Support adding items to a list or map

[auke-]

The native Java generator produces specific methods for adding items to a list and putting items in a map (see https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/Java/libraries/native/pojo.mustache#L127). This is useful when working with programmatically created model instances since all their properties are initialized with null.