[WIP] feature: Support for multiple PKI infrastructures

[mlgiraud]

This PR adds support for multiple PKI infrastructures. In OPC UA a PKI infrastructure corresponds to a CertificateGroup. A CertificateGroup has an issuer authority that issues certificates. This PR adds support such that a server can belong to multiple different CertificateGroups. Endpoints can be assigned to a specific CertificateGroup.

These features require changes to the public and private key loading and to some security related code.

[andreasebner]

@mlgiraud Thank you for the PR! It would be great if you can add a small comment to support others to understand your changes ;)