Add metrics for certificate expiration date

[MartinWeindel]

What this PR does / why we need it:
A new metrics named cert_management_cert_object_expire has been added. It provides the expiration date as Unix time (the number of seconds elapsed since January 1, 1970 UTC) for each Certificate object.

If the metrics is collected by Prometheus, the remaining days before expiration can be calculated with PromQL expression

(cert_management_cert_object_expire-time())/86400

Which issue(s) this PR fixes:
Fixes #130

Special notes for your reviewer:

Release note:

Added metrics named `cert_management_cert_object_expire` for certificate expiration date.

Updated golang builder image from version `1.20.4` to `1.20.5`.