[META] Add methods to perform common core administrative actions

The jsonapi_core submodule is a little spartan. Ideally, it would support most if not all of the common operations that one performs to administer a site.

These operations include, but are not limited to:

• Clear cache
• Run cron
• Enable/disable maintenance mode
• Import/export/sync configuration
• List available extensions and their status
• Enable/uninstall extensions
• Rebuild node access permissions
• Clear the watchdog log
• Flush image styles
• Reindex the site's search index

This issue is here to track the progress of implementing these methods. As each one is taken on, a child issue should be created and this issue summary should be updated with a link to the child issue.

The above list is not exhaustive. If more are found, they may be freely added.