Rasa

- [10634](https://github.com/rasahq/rasa/issues/10634): Add `RASA_ENVIRONMENT` header in Kafka only if the environmental variable is set.

Miscellaneous internal changes
- [10643](https://github.com/rasahq/rasa/issues/10643)

- [10621](https://github.com/rasahq/rasa/issues/10621): Make `rasa data validate` check for duplicated intents, forms, responses
and slots when using domains split between multiple files.

Bugfixes
- [10623](https://github.com/rasahq/rasa/issues/10623): Add an option to specify `--domain` for `rasa test nlu` CLI command.

- a small bugfix to domain merging.

- [10137](https://github.com/rasahq/rasa/issues/10137): Fixes Domain loading when domain config uses multiple yml files.

Previously not all configures attributes were necessarily known
when merging Domains, and in the case of ``entities`
were not being properly assigned to ``intents``.

- [10471](https://github.com/rasahq/rasa/issues/10471): Fix load method of LexicalSyntacticFeaturizer to make the class inheritable.
- [7645](https://github.com/rasahq/rasa/issues/7645): Do not print warning when subintent actions have response.

- [10413](https://github.com/rasahq/rasa/issues/10413): The value of the `RASA_ENVIRONMENT` environmental variable is sent as a header in messages logged by `KafkaEventBroker`.
This value was previously only made available by `PikaEventConsumer`.

Bugfixes
- [10458](https://github.com/rasahq/rasa/issues/10458): Make `action_metadata` json serializable and make it available on the tracker. This is a backport of a fix in 3.0.0.