Support reading keys from systemd credentials

[squalus]

Add support for reading the secret key and the session key from the following files:

• $CREDENTIALS_DIRECTORY/secret_key
• $CREDENTIALS_DIRECTORY/session.secret

This allows using the systemd LoadCredential directive to handle keys. For example:

LoadCredential=secret_key:/var/lib/standardfile/secret_key.txt
LoadCredential=session.secret:/var/lib/standardfile/session_secret.txt

Tested using both hardcoded secrets and LoadCredential files.

[squalus]

Committed both code changes, and added some notes to the example standardfile.yml config about using systemd to pass credentials.