Re opening #1712 to push up.

As a user, instead of keeping raw variables (which may be secrets) in my workflows, I would like to be able to reference a secret using " {{ secrets.SECRET_NAME }}".

Think about a use case, where I use webhook provider and I want to add api key header: today it has to be plain text in the workflow

PRD:

New screen in the workflows/specific workflow page, that allows one to CRUD tenant secrets.
New API that utilizes SecretManagerFactory.get_secret_manager to CRUD secrets in the backend
Load relevant workflow secrets to context_manager to allow reference in a workflow