Implement SAML Interface (TR-03130 Part 1)

[Hermann-Core]

Implement the SAML-based eID-Interface endpoints for authentication and attribute retrieval.
This includes SAML 2.0 AuthnRequest parsing, Assertion generation, XML Signature/Encryption, metadata exchange, and binding support (HTTP-Redirect, HTTP-POST, SOAP/PAOS).

Estimated time: 1 week

[Ogenbertrand]

This is going to be on hold for now.