Include date of enrollment for oxExternalUid

> can we send along the date of enrollment in the metadata?
> otherwise we’re not able to give the date added in oxTrust.. should also be displayed in Casa
> its a helpful piece of data


This affects passport_social, passport_saml, and casa account linking plugin

Changes should not disturb otp/hotp since that will require modifications in otp script, otp casa script, and casa itself (enrollment timestamps for otp are already stored elsewhere).