Add check for self-signed certs

[starbops]

The newly added function will detect whether the user-provided ssl-certificates Setting contains a self-signed certificate. If the certificate was not generated and signed properly, e.g., include the right IP SANs, an error will occur in the following upgrade.

Related issue: harvester/harvester#4519

[w13915984028]

Please also check this new issue: https://github.com/harvester/harvester/issues/4531

[starbops]

Close this as it becomes unnecessary. We've already shipped the fix in the new version (harvester/harvester#4585) and have the workaround covered in the docs (harvester/docs#462).