Optional interface for JIRA services that are backed by plugins. Due to the nature of the plugin system, a service
that is backed by a plugin may become unavailable at any time if the plugin is disabled or uninstalled.
Returns true if this service is usable. By default a service is always usable, but services that are backed by
plugin jobs, for example, may become unusable when the plugin is uninstalled or disabled.
Returns:
a boolean indicating whether the underlying service is usable