3.0 slot mappings: rasa-sdk implementation

[ancalita]

Description of Problem:
Background: This is provided in Taking Apart Forms and Slot Mappings.

Summary: The approach of filling slots in 2.0 includes an implicit mechanism (via auto-fill) and an explicit mechanism (via forms). This makes it challenging to keep information up-to-date throughout the course of a conversation, as well as requires slot mapping duplication across forms if multiple forms should fill the same slot.

Overview of the Solution:

• The full extent of proposed changes can be found in the implementation proposal.
• This issue is only concerned with the section requiring adaptations to the rasa-sdk repo.

Definition of Done:

• adapt FormAction to mirror changes in rasa-oss FormAction
• adapt FormValidationAction interface
• changes are unit tested