Allow/document configuration via env vars

Currently it seems to be required that configuration is done with command line arguments (e.g. `--service-name=pod-identity-webhook`), it would be useful if the daemon also looked at environment variables (e.g. `AMAZON_EKS_POD_IDENTITY_WEBHOOK_SERVICE_NAME=pod-identity-webhook`)

Each command line argument should have a matching env var, which is uppercased, `-` swapped for `_`, and prefixed with e.g. `AMAZON_EKS_POD_IDENTITY_WEBHOOK_`.

Start by locating the daemon's command-line argument definitions and configuration initialization. Check how each argument is parsed, then verify that every argument has the specified prefixed environment-variable equivalent and that configuration from both sources is covered by tests.

Written by the indexing model from the issue text.