extract lib for main core functionality

[shazron]

Core functionality: https://github.com/adobe/aio-cli-plugin-certificate/blob/master/src/certificate.js

This is because a lot of libs are using this plugin to use this code, when this package is really a plugin and ui for the aio-cli.

Dependents: https://www.npmjs.com/package/@adobe/aio-cli-plugin-certificate?activeTab=dependents
This results in a situation where a lib is dependent on a plugin.

[aiojbot]

JIRA issue created: https://jira.corp.adobe.com/browse/ACNA-2713