Refactor: Garm uses the generalized `cert-reloader` once implemented

[mlajkim]

Background

To implement an X.509 readable Garm to replace the deprecated ntoken,
we've designed Garm to use the cert-reloader for reading local X.509 certificates and their keys.

Since there was no generalized library available,
we directly copied the code from athenz/k8s-athenz-sia. You can view the implementation with TODOs here:
https://github.com/AthenZ/garm/blob/master/service/cert-reloader.go#L15-L20

What should be done?

• Bring the generalized cert-reloader once implemented