Handle OIDC prompt together with SAML ForceAuthn and IsPassive

[melanger]

Allow processing prompt=none/login/select_account/consent, ForceAuthn=true and IsPassive=true, including interoperability (e.g. IsPassive=true from SAML frontend is converted to prompt=none in OIDC backend)

Change is backward compatible considering nobody is using prompt and IsPassive on frontends or the constant Context.KEY_FORCE_AUTHN.

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