Add option --accessible to fedcloud site save-config to save only sites accessible for users

[tdviet]

Users don't need to save the sites that are inaccessible to them. Let them save only the sites that are relevant. That can speed up all-sites operations and provide cleaner output. Access tokens or OIDC account will be needed for checking access.

[tdviet]

The same option --accessible for fedcloud site list to list only sites accessible to users, not all sites.