Support SEID2 server certificate

The library now supports the SEID2 profile when validating the Posten signering server certificate when establishing secure TLS connection.

Posten signering has not yet migrated to a new SEID2 enterprise certificate, but this will eventually happen, and clients must include the new SEID2 trust hierarchy and be able to extract the organization number which is embedded differently in SEID2 certificates. The library now uses the implementation from the Digipost certificate-validator to extract the organization number.