[WIP] Read Cloud client/secret from environment variables

[manchoz]

Motivation

Allows to retrieve Arduino Cloud API Tokens from environment variables

Change description

• If environment variables do exist, overwrite the ones provided in the config file.
• Do not exit if the config file does not exist and environment variables are set.

Additional Notes

Reviewer checklist

• PR address a single concern.
• PR title and description are properly filled.
• Changes will be merged in main.
• Changes are covered by tests.
• Logging is meaningful in case of troubleshooting.
• History is clean, commit messages are meaningful (see CONTRIBUTING.md) and are well formatted.