Add feature flags for in development features for the first release

[smcclure20]

For the initial release, we will feature flag some features, especially those that have not been as thoroughly tested and/or do not exist on all clouds.

In particular, we will flag the following:

• Attach Resource
  • Current state: only implemented for Azure and not well-tested.
• Kubernetes Clusters
  • Current state: implemented for all clouds, but not well-tested.
• Private Endpoints
  • Current state: implemented for all clouds, but not well-tested.

To implement the feature flags, we will add flags to the controller config. The orchestrator will report a warning to users as they use flagged features.