Add ability to validate SAML response against identity provider certificate

The following web tool provides the ability to validate a SAML response digital signature against the identity provider's public certificate. It would be nice to add this functionality into this tool.

https://www.samltool.com/validate_response.php

[This page](https://stackoverflow.com/questions/21209510/validating-saml-signature-in-python) talk about [`xmldsig`](https://github.com/AntagonistHQ/xmldsig) as a tool that may already be able to do this.