better method of syncing known_hosts file

When uploading ssh data, we should be able to sync the known_hosts file so that known hosts can be saved across different machines.

details:

support storing and writing known_hosts
expose KnownHosts in DataDto
sync known_hosts when downloading or uploading
document this feature in README

ssh-sync-server is being updated in this PR https://github.com/therealpaulgg/ssh-sync-server/pull/31