[FEATURE] Create REST API calls to add, update, or remove an extension

[dbwiddis]

Is your feature request related to a problem?

Part of #356

What solution would you like?

Create REST API calls to add, update, or remove an extension from the system index developed in #357

• Update the index
• Optionally allow user to download updated extensions.yml file
• Stop the extension when removed; reboot extension when updated
• Start up an extension on local node if so configured
• Give user instructions on starting an extension on a remote node