Adjustment to new issuer

[auhlig]

The interface for an external issuer to the cert-manager changed.
Previously, it was required to specify the group, version and kind (unless defaulted) for certificates. This is no longer the case. Furthermore, custom kinds are not allowed.
We need to adjust our CRDs to be of kind issuer and/or clusterIssuer and the corresponding implementation in the controller.
Without specifying the full issuer reference (group, version, kind) in a cert-manager certificate various failures might occur.
Thus, the issuer is no longer productively usable with the latest cert-manager as we use kind: DigicertIssuer. See here.

Also the approval conditions should be supported.
Bonus points: Request to be added to the list of issuers to increase visibility.

[auhlig]

Reworking to adjust to the newer external issuer interface is quite an effort.
We're currently looking into a pragamatic alternatives such as ACME-based issuers for DigiCert and ultimately deprecating this project.

[auhlig]

@kayrus took on the task of trying the ACME solution.
Unfortunately, it's not a viable solution at the moment and we need to stick with the digicert-issuer.
Support for recent cert-managers was added via #10 and is part of release v2.2.0.

Further cleanup might be necessary but is currently not a priority.