feat(rsyncd) allow specifying path to existing host keys through configuration.sshd.persistentHostKeys

[dduportal]

Related to jenkins-infra/helpdesk#4402 (comment)

Each time the rsyncd (with sshd enabled) restarts on a new node, it regenerates the SSHD host keys which fails any new connections.

This PR updates the chart (and entrypoint - ref. jenkins-infra/docker-rsyncd#29) to allow specifying a directory in which to get existing SSH host keys at restart through the configuration.sshd.persistentHostKeys value.

Tested locally on k3s by using a component's specified PVC with a subdir:

• Installed the chart one time without the persistence key enabled
• Copied the generated keys in the PVC's subdir
• Ran one time a rsync command: it asked the usual "yes/no" question about host fingerprint. Accepted tyo have my known_host up to date.
• Uninstall the chart (except the PVC)
• Re-install with configuration.sshd.persistentHostKeys specified to the PVC's subdir
• Ran a second time the rsync command: no host fingerprint change detected!