CLI: rethink step validation

[josephjclark]

The CLI has logic to validate the structure of a workflow.

The idea here is that if a user mis-types a key like "adapatator", the CLI will catch the error and flag them.

What with all the focus on sync and portability, it's way more likely that a workflow will be machine generated now, and so won't have those typos.

But if the app adds a new key to the workflow, the CLI will refuse to execute it - even if it's a step it doesn't care about.

I would like to think about the following options:

• Use the common portability spec schema to validate against. The app does have a JSON schema. If the workflow or owning project has aversion, use that, else use latest. This relates to Better coupling of portability spec with kit lightning#4734
• Add a --validate flag which will validate a workflow for you, if you want it (a bit like a circle CI yaml linter app)
• only validate keys that the CLI cares about, and ignore the rest