This PR consists of two parts aiming for the same goal: Better access control and auditing capabilities.

1. Instead of one user/pass which identifies the certificate, users can now share certificates. You can define multiple credential pairs for different teams/departments sharing the same certificate.
2. The usage of the service is now tracked. While we do not store the details of each and every requests semantically, the service stores who used which certificate how often. This allows taking decisions based on the usage of the certificates.

As this is a breaking change, version was bumped to 3.0