[FEATURE] Create a system index for tracking extensions

[dbwiddis]

Is your feature request related to a problem?

Part of #356

What solution would you like?

Create a system index for tracking extensions

• store data corresponding to the extensions.yml file
  • establish functionality to import/export to synchronize this file with the index
• allow option for this index to be priority for persistent extensions list rather than config file
• Use this index rather than the existing maps/lists in ExtensionsManager