Update workflow schema

Current situation

The Workflow schema has been stable for a long time. Some new needs have surfaced (timeout for a workflow, insights, workflow hooks, ...) that cannot be cleanly handled without upgrading the schema.

Desired outcome

• Add a timeout-minutes option (an integer or an expression) at workflow level
• Add a strategy option at workflow and job level
• Allow for expressions in more places
• Allow for inputs and outputs at workflow level
• Allow for secrets at workflow level
• Allow for concurrency at workflow and job level