sshguard: T5354: Add service ssh dynamic-protection

Sshguard protects hosts from brute-force attacks
It can inspect logs and block "bad" addresses by threshold
Auto-generates own tables and rules for nftables, so they are not
intercept with VyOS firewall rules.
When service stops, all generated tables are deleted.

set service ssh dynamic-protection
set service ssh dynamic-protection allow-from '192.0.2.1'
set service ssh dynamic-protection block-time '120'
set service ssh dynamic-protection detect-time '1800'
set service ssh dynamic-protection threshold '30'