Xray should give the possibility skip custom fields update when being enabled

When enabling Xray, the Xray custom fields are either created or updated. Most of the time, the update operation won't change anything on the custom field and yet, it still consumes CPU time.

Xray should provide a dark feature (https://confluence.atlassian.com/jirakb/enable-dark-feature-in-jira-959286331.html) where users can enable/disable the update of Xray custom fields.

This dark feature is called: com.xpandit.raven.skipCFUpdateOnPluginEnable

If this feature is enabled, Xray will skip the custom fields update.