Feat: set flags via rill.yaml

[briangregoryholmes]

This PR enables the setting of feature flags via the rill.yaml file and updates the formatting expectations of localStorage keys to match the YAML and URL params.

• Adds a "features" key to the YAML spec that takes a list of strings
• Updates the FeaturesFlag store to use the new class based structure
• Adds a subscriber function that listens for changes to the rill.yaml and updates keys accordingly
• Adds a "system" or "user" scope to limit who can change feature flags
• Updates rillYAML and RillYAML types in the platform code

As an example, if you add (or remove) the following in your rill.yaml:

features:
  - customDashboards

The UI should respond immediately (after a debounce) to toggle that feature on or off. Adding something like adminServer would not enable that feature as this flag has been created with system scope, but no warning is surfaced.

A flag needs only to be set in one location to be enabled, so if you have customDashboards set in the URL, localStorage and YAML, then it won't be turned off in the UI until it's removed from all three locations.

While this will not be a widely used feature, I would argue that we should remove the ability to set flags in the URL/localStorage now that it's enabled in the project YAML.

[nishantmonu51]

LGTM, 👍