Provide Optional Param for SAML Auth

[MarkDordoy]

This PR provides a way to set the EntityID. This can be very handy when you have multiple applications being developed against a single IDP. IDPs I've worked with always required the EntityID to be unique as a way to identify which relying party/Enterprise app should be used when performing authentication.

[palantirtech]

Thanks for your interest in palantir/go-baseapp, @MarkDordoy! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.