Add a `git-bundle-server` command or installed script for generating auth configs

[vdye]

Following up on this comment: we require fixed auth to supply the SHA256 hash of the password to avoid saving it in plaintext on disk. To provide a slightly more user-friendly experience generating this (and other) auth configs, we should create an interactive command or script to configure the desired auth config.

An example output might look like:

$ git-bundle-server generate-auth-config
Which mode would you like to configure for (not case-sensitive)?
* fixed
* plugin

Answer: fixed

Please specify a username: admin
Please specify a password:

Your auth config was written to './auth-config_20230511_160023.json'.
$