[Fleet] Custom component template enhancements

[jen-huang]

From the discussion in the original implementation of custom component templates for user settings (#101769): there are a few items we should think about further:

• Whether or not packages should specify if a custom template is installed or not: from discussion with @ruflin and @mostlyjason, it would be rare for a package to not want this (Endpoint seems to be the only case), so we can add an opt-out option later on.
• Whether or not the custom template should be deleted when a package is uninstalled: let's wait for user feedback on whether deleting it automatically is an annoying UX. In the future we could potentially expose a checkbox/flag to delete custom settings on uninstall.