Allow pangolin-cli to read machine credential settings from config file

[rskallies]

Summary

Currently when run the client on a headless machine the --id --secret and --endpoint option must be used. This is also true when using a systemd unit. Please add a way to read these from an environment file. Currently any user on a system can see the credentials in the process list.

Motivation

Security best practices.

Proposed Solution

Add read from env e.g. /etc/default/pangolin-cli or systemd.service with Environment=SECRET=xxx or EnvironmentFile=/etc/default/pangolin-cli

Alternatives Considered

No response

Additional Context

No response

[rskallies]

I'm now using Olm CLI instead of the Pangolin CLI. Olm generates a config file on first launch, after that it reads from the file. So this must be 'advanced usage' according to the docs [https://docs.pangolin.net/manage/clients/configure-client#olm-advanced]

So I think this feature request is already fulfilled.