config: firewall: add types for log level and rate limit

config: firewall: add types for log level and rate limit

Adds types for log and (log-)rate-limiting firewall config options as
well as FromStr implementations for parsing them from the config.

Co-authored-by: Wolfgang Bumiller <w.bumiller@proxmox.com>
Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
Reviewed-by: Lukas Wagner <l.wagner@proxmox.com>
Reviewed-by: Max Carrara <m.carrara@proxmox.com>