Add validation based on identity provider

Each identity providers uses metadata to integrate with a service provider. The formation of these metadata tend to follow a pattern for a given IdP. Given that to be true, the sending IdP could potentially be identified from looking at SAML metadata, such as:
- Their issuer URI (EntityID)
- Their single sign-on URL (not sure if this comes in the SAML Response, but definitely comes in the Request)
- Their X.509 certificate subject

On the other side of that, if the identity provider is known, we can use that patterning to check for errors in a received SAML response.

This would require a corpus of data for common identity providers, so some research will need to be done in order to implement this feature.