Make omniauth-saml federation compatible

[TheRealKingS]

The Gem is used by GitLab to enable SAML authentication there. However, there is the serious disadvantage that the Gem cannot handle federations (DFN AAI, eduGAIN, InCommon etc.). Each identity provider must be configured manually. Is there a way to make the Gem federation ready? With particular regard to Shibboleth, which is used a lot especially in the education sector.