[config] Pre-checks for ~/.sky/config.yaml

[Michaelvll]

Currently, we don't have a way to pre-check the ~/.sky/config.yaml, making it very easy to miss spell or setting wrong configs in the yaml. We can apply the json schema check to the ~/.sky/config.yaml to make sure there is no unexpected settings.

A user met the issue when specifying the spot controller's resources requirement, but missing one section (resources):

spot:
  controller:
    cloud: gcp
    region: us-west1

[djokester]

Can I pick this up?

[concretevitamin]

#2600 ran into this.

[iojw]

Let me know if you're still interested in picking this up @djokester! Happy to guide you.