fix(ssh): verify the host key netOrk expects, report the one seen (NetOrk/netork#1167)

The driver accepted any SSH host key. It now verifies through
napalm-device-types' HostKeyCheck (>= 4.19.0): with a known key in
optional_args["ssh_host_keys"] any other key is refused before login, and
every key seen is reported to optional_args["ssh_host_key_observer"].
Without either argument it connects as before (first use).

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Security
+- The node's SSH session (the fallback for node commands and the provisioning steps) verifies its host key (NetOrk/netork#1167): the key the caller expects for port 22 in `optional_args["ssh_host_keys"]`, else the first one; every key seen goes to `optional_args["ssh_host_key_observer"]`. A changed key is refused before any login: a provisioning step fails, a best-effort read returns nothing. Both paths now share one session (`_node_ssh`). Replaces paramiko's `AutoAddPolicy`. Requires napalm-device-types 4.19.0; version 0.5.0.
+
 ### Changed
 - **Breaking:** `get_device_warnings` is gone; the node reports facts and the caller
   names the warnings (netOrk #1111). lldpd and the pending updates were already facts