Support systemd credentials out-of-the-box

[muety]

Systemd version 247 introduced credentials management, and version 250 made it even more convenient to use.

They suggest that:

[...] In an ideal world, well-behaved service code would directly support credentials passed this way, i.e. look for $CREDENTIALS_DIRECTORY and load the credential data it needs from there. [...]

This way, passwords, etc. wouldn't even have to be passed as env vars to the process. I'd like to support that in Wakapi. Env var would still take precedence if set, though.

Moreover, the example wakapi.service file should contain an example that uses these managed credentials for database credentials and password salt.

Something like:

LoadCredential=db_password:/etc/wakapi/db_password.txt
Environment=WAKAPI_DB_PASSWORD=%d/db_password

Version 250 doesn't seem to be as wide-spread, yet though.

Also, include a systemd-creds command example on how to create new (encrypted) creds.