Feature request: Add DB_PASSWORD_FILE or similar

[Munksgaard]

I am looking to set up pganalyze on my Nix-based system. Unfortunately, due to the way I am handling secrets, I cannot directly provide the password in an environment variable (that would require storing the value in the nix store, which is readable by anyone with access to the system), so I was wondering if you might consider adding support for a DB_PASSWORD_FILE environment variable or similar. This would let me put the password file in a secure place (only readable by the pganalyze user) and then pass the file location to the program.

Thanks for your work on pganalyze, I'm really looking forward to trying it out on my system!

[Munksgaard]

For the record, I've created a Nix flake that makes it easy to install the collector with Nix.