[PYTHON] GetItem not working for Client generated allOf model and broken since 5.2.0 (#12239)

* I feel the issue is due to the creation of self._var_name_to_model_instances while doing the deserialization of the data.

Earlier the Python SDK code was using get_var_name_to_model_instances function which was adding var name to model instances that contain it. So <class 'openapi_client.model.stream_options_all_of'> will not part of mapping in self._var_name_to_model_instances for variable name stream_options.

Now with the latest Python SDK code following is the way through which var_name_to_model_instances is created:

    for prop_name in model_args:
        if prop_name not in discarded_args:
            var_name_to_model_instances[prop_name] = [self] + composed_instances
Now as we can see that the var_name_to_model_instances is populated with self and composed_instance which will also contain stream_options_all_of as a composed instance and there will be no check that if stream_options is present in composed_instances or not.

As there is no attribute_mapping found for stream_options in stream_options_all_of, the type for stream_options will be treated as dict for mapping stream_options_all_of as mentioned by @Chekov2k.

So what I suggest is the following code:

    for prop_name in model_args:
        if prop_name not in discarded_args:
           var_name_to_model_instances[prop_name] = [self] + list(
                filter(
                    lambda x: prop_name in x.openapi_types, composed_instances))
This way we can check if the property name is present in that composed instance or not. If it's okay for @spacether I can raise a PR for this.

* [get_item_all_of_bug]
Added samples, test cases to validate all_of schema.

* [getiem_all_of_bug]

Updated docs and samples.

* [getiem_all_of_bug]

Updated test cases, docs and samples.

samples/openapi3/client/petstore/python/docs/StreamOptions.md

@@ -0,0 +1,13 @@
+# StreamOptions
+
+
+## Properties
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**egress_thresholds** | **{str: (float,)}** |  | [optional] 
+**publish** | [**PublishOptionsPublish**](PublishOptionsPublish.md) |  | [optional] 
+**any string name** | **bool, date, datetime, dict, float, int, list, str, none_type** | any string name can be used but the value must be the correct type | [optional]
+
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+