feat: add support for reloading certs when renewed

[afrancis101]

For #45 so the k8s-image-swapper can handle the rotation of webhook certificates

[estahn]

Would you mind explaining how this works? Under what conditions is "getCertificate" executed?

Once a Certificate is returned it should not be modified.

[afrancis101]

Would you mind explaining how this works? Under what conditions is "getCertificate" executed?

Once a Certificate is returned it should not be modified.

Sure, my understanding is that it is executed on every TLS handshake. Which does makes it pretty inefficient at present!
One of my colleagues has provided an example of caching the cert/key response and only updating the response if the files change. I'll look to update the PR to use that pattern instead if that's okay?

[afrancis101]

Once a Certificate is returned it should not be modified.

On this, I'd argue that the returned certificate is not being modified. The cert/key on file will change over time but we are not modifying what is returned to GetCertificate. Other projects such as kubernetes-sigs/controller-runtime use similar patterns from what I can see too.