Add Extension API

[ncdc]

This will be a namespace-scoped API.

Spec fields:

• paused
• serviceAccountName
• defaultNamespace
• source
  • direct
    • url
  • package
    • name
    • version
    • channel
• upgradeConstraintPolicy

[ncdc]

@tmshort was chatting with @joelanford and @varshaprasad96 and we've tweaked the spec fields a bit around sourcing from a catalog or a direct bundle reference. spec.source.direct and .package are new/renamed and are mutually exclusive.