write export tool to automatically pull password hashes from keycloak

Moved from: #50 and #210 (closed)

Project repo: https://gitlab.archlinux.org/archlinux/mail-credential-syncer

Since many members of arch-devops work with rust and it's good security characteristics, this tool shall be written in rust (#210 (comment 6535) contains a very dirty POC in Go).

Three config parameters:

• Path to mapping file for keycloak UUID -> arch mail address on local FS
• Keycloak hostname
• Post-receive script

Implementation:

• Use inotify to receive events when the mapping file changes
• Subscribe to pw change events for the keycloak user attribute mail_password_hash

Whenever an event fires:

• iterate over mapping
• get pw hash from keycloak
• check if the hash is valid and contains no malicious input (probably with a regex)
• export dovecot and opensmtpd version of virtual user file
• backup old config files
• run post receive script
• when the post receive script failes, restore old config files
• report error via e-mail (or prometheus?)