Implement alternative ways for configuring database credentials

[alco]

Currently, the only way to set the username and the password for database connections is via the DATABASE_URL environment variable. This is insecure and may not be allowed in certain runtime environments. Electric should allow to configure secret credentials separately from the connection info, such as by reading it from a file on disk or on an in-memory file system.

Motivated by #2149. Related issue - #1885.