Feature: Regex support in --secret-type flag

[AnuragEkkati]

New Feature Request:

We use x509 extensively in our production to monitor the certs. Recently during onboarding one of our internal customer, we realized that it is a hassle to keep adding the flag --secret-type for every custom key of the secret where the value is certificate we want to monitor.

It will be very helpful if we have regex support like below

--secret-type=Opaque:.*.crt --secret-type=Opaque:.*.pem

If x509 exporter has to scan all of the cluster secrets and if we have too many customizations in the secret keys it becomes unmanageable

[AnuragEkkati]

@npdgm kindly take a look at the PR #243