Add support for systemd's LoadCredential mechanism

[Tom-Hubrecht]

C.f. https://systemd.io/CREDENTIALS/

This allows using systemd's native secrets management mechanism by default

[hyperlint-ai]

PR Change Summary

Added support for systemd's LoadCredential mechanism to enhance secrets management capabilities.

• Introduced loadcredential_settings parameter in multiple settings classes.
• Updated return statements to include loadcredential_settings for better integration with systemd's secrets management.
• Enhanced documentation to reflect the new LoadCredential support.

Modified Files

• docs/index.md

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[hramezani]

Thanks @Tom-Hubrecht for creating this PR. Unfortunately, we can't accept these new settings source because of the following reasons:

• People are not interested in this settings source, and this is the first request.
• pydantic-settings provide the ability to have this settings source as a custom source in your code
• It is really hard for us to maintain these settings sources, and we are already maintaining a lot of them.

Thanks for your understanding.