Make it possible to OR slot values - Implementation

[joejuzl]

Description of Problem:
Background: #7715
Proposed Solution: https://www.notion.so/rasa/Make-it-possible-to-OR-slot-values-4abdf1b6632c4bb2881d2057b58ca335

Summary:
We want to be able to use or for slot_was_set steps in stories.
It currently only works for intent steps.

Todo:

• updating the story schema rasa/shared/utils/schemas/stories.yml
• updating YAMLStoryReader
  • _parse_or_statement
• updating StoryStepBuilder
  • adding a slot_was_set equivalent to add_user_messages
• make sure rasa.shared.core.training_data.visualization is updated for Rasa X story visualisation

Example Story:

- story: tell name bob or joe
  steps:
  - intent: greet
  - action: utter_greet
  - intent: tell_name
  - or:
    - slot_was_set:
        - name: joe
    - slot_was_set:
        - name: bob
  - action: utter_generic_hi

Definition of Done:

• or functionality added for slot_was_set events
• Rasa X visualisation compatibility
• Tests are added
• Feature described the docs
• Feature mentioned in the changelog