[FEATURE] Support kubernetes secrets as a fallback

[stephenlacy]

I implemented a basic version of using kubernetes secrets as a fallback method here: stephenlacy@56e6cd1

Usage in a docker container:

ENTRYPOINT doppler run --kubernetes-secrets-fallback --kubernetes-namespace app -- "/app/bin/$PACKAGE"

The main premise is on a kubernetes cluster the pods are auto-scaling, as such they can and will restart at any time. If for any reason doppler services are unavailable then 100% of the restarting pods will fail and will cause a crash loop.

Not sure if there are plans of supporting fallbacks as modules, if so having an AWS KMS and kubernetes secret fallback would be interesting.