[SAML] Allow to skip the IDP selection
The goal is to be able to select an IDP in the AuthSAML module, so a direct authentication can be made from the IDP to the SP.
It is a kind of IDP initiated SSO, but without the need to generate a SAML AuthnResponse. The idea is to pass a parameter to the SP indicating the IDP to use, so SP will directly create the AuthnRequest to the pre-choosen IDP.