feat(feature_flags): added support for basic feature flag management

[Arqu]

Adding feature flags management to qri core. Cloud should reuse this with a different store backing it, and frontend should source flags from the /config/flags endpoint.

It's a very basic FF system to allow us to hand roll and expand as we need. It's pretty open and allows us to add our own flags while developing without much fuss. Once we deem a feature needs to be really feature flagged we add it to the list of defaults.

[Arqu]

Just distilling come notes from our discussions around this:

• we should rename this to capabilities
• it's intentionally loose compared to config
• we're prioritizing cloud/frontend usage with this currently
• we should be weary of the code AND business complexity this adds long term
• the client/user of the flags should hold the responsibility to look and understand the flags that are relevant to it

• the overall response shape should look something like

  {
  "core.p2p": true,
  "core.automation": true,
  "core.email": false,
  
  "automation.emailCompletion": true,
  
  "experimental.issue.comments": false
  }

@b5 @ramfox @dustmop @chriswhong tagging you to add in feedback and help shape this

[Arqu]

Closing due to age. Will redo once we need it.