Action Providers

Currently, actions have to be registered in Mistral's SQL DB and has to have a corresponding class module installed as entry_points (regardless whether the class is a stub that serves as proxy or a class that does actual work). ActionProvider is an abstraction that allows a trusted third party system to extend Mistral with its own action catalog and action executor without the heavy lifting in keeping the action table in sync and in creating action generators and maintaining the class stubs. The high level architecture is attached for reference. The ActionProvider allows for action executions from specific providers to be isolated either for performance and security reasons. The architecture also allows ActionProvider to be independently scale up/down based on demand. With ActionProvider, Mistral can delegate the action catalog and action execution to the providers and focus primarily on managing the workbooks, conducting the overall workflow executions, and tracking execution state.

TODO: add link to the spec and more details