Select IdP by alias

[arykov]

This applies to security-saml2-core version 2.0.0.M30.
Currently SamlAuthenticationRequestFilter only allows selection of Identity provider by its entityId, dictated by metadata. It is fine when done through UI, but in most cases applications have only one identity provider and configure using
loginPage("..../discovery?idp=");

It would make way more sense to do this by alias which is in control of service provider unlike entityId dictated by Identity provider.