Refresh TLS certificates automatically

[uhthomas]

Describe the bug

Athens supports TLS, but will only read the certificates once on startup. Environments like Kubernetes with cert-manager may automatically rotate the given certificate, where Athens will continue to use the old certificate. There are numerous linked issues:

• Facilitate ConfigMap rollouts / management kubernetes/kubernetes#22368
• docs: Add info about client side certificate rotation best practices. cert-manager/cert-manager#1168

It may be better for Athens to support this natively. There seems to be a good Go package which facilitates this behaviour:

https://github.com/cloudflare/certinel

Error Message

N/A

To Reproduce

N/A

Expected behavior

Athens should use new certificates if available.

Environment (please complete the following information):

• OS: Linux (Kubernetes)
• Go version: N/A
• Proxy version: v0.12.1
• Storage: N/A

Additional context

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