Support hot reloading of TLS certificates

[camillsir]

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• I'd be willing to implement this feature (contributing guide)

Describe the user story

I'm currently using atlantis in a kubernetes cluster. I'm using a product called cert-manager to generate Let's Encrypt certificates. These certificates are short lived and cert manager will handle generating a new certificate and updating the certificate file. Atlantis currently only reads the TLS certificate on start.

Describe the solution you'd like

To accomplish TLS certificate rotation without restarting the application, atlantis will need to monitor the certificate path and reload the TLS certificate when the file has been modified.

Describe the drawbacks of your solution

The solution assumes that the modification (mtime) of the file changes when the certificate is updated. This works in my case, but might not work for others.

Describe alternatives you've considered