Support dryRun feature when creating or editing resources

[richard-cox]

Is your feature request related to a problem? Please describe.

• Creating or editing a resource can run through various steps within Kube that can make the request fail
• the dry run concept can be used via kubectl or api to determine if the request would be accepted and succeed
• For docs see https://kubernetes.io/docs/reference/using-api/api-concepts/#make-a-dry-run-request

Describe the solution you'd like

• Add an option on create/edit to 'dry run'
  • only for required types
  • should not show resources outside of kube api
  • could probably do both of above via a flag on the steve or specific resource models
• Show the result of the dry run (either success or the error message)

Additional context

• all core types should support this, however it's not for sure all resources (include CR) will always support as well. We need to decide whether to just do this for core types or find a way to determine if we can dynamically tell if a resource supports it
• also need to decide whether to use dry run against admission controllers (may help the above case)
• other http verbs also support it, we should consider as a separate task whether to integrate with those too

[richard-cox]

There's a wider discussion on improving client side validation, which might supersede this, but this feature could be a low bar stop gap until then.