Remove direct calls to xmlseclib

[baszoetekouw]

Most of our SAML/XML handling is done via the SimpleSAMLphp/SAML2 library. But in three places, we still call xmlseclibs directly:

• https://github.com/OpenConext/OpenConext-engineblock/blob/main/src/OpenConext/EngineBlock/Metadata/X509/X509PrivateKey.php#L64
• https://github.com/OpenConext/OpenConext-engineblock/blob/main/src/OpenConext/EngineBlock/Xml/DocumentSigner.php#L46
• https://github.com/OpenConext/OpenConext-engineblock/blob/main/library/EngineBlock/X509/PrivateKey.php#L45

Investigate whether we can replace these with functionality of SAML2.

The advantage of that is that we have a single point of entrey into xmlseclibs, and because SAML2 is implementing additional safeguards against wrapping attacks and such, it makes us a little more robust against xmlseclibs/libxml bugs.