Add EGI Checkin (Keycloak) token integration

[sashakames]

@djspstfc hopes optimistically that the Globus Auth (OIDC) integration can be reused for this task. We need an assignee from CEDA for this task (and add them to the ESGF org if not already)

[djspstfc]

Assigning myself but work will be done with Rhys