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 SSH host key is verified (NetOrk/netork#1167). `open()` hands netmiko the key the caller expects for the port in `optional_args["ssh_host_keys"]` (as a `known_hosts` file that exists only while it connects, with `ssh_strict`), so any other key is refused before the login (`SshHostKeyMismatch`); without one netmiko accepts the first key. Every key seen goes to `optional_args["ssh_host_key_observer"]`. OpenMediaVault and QNAP get this through `LinuxDriver.open()`. Requires napalm-device-types 4.19.0; version 0.11.0.
+
 ### Changed
 - **Breaking:** `get_device_warnings` is gone, and with it the `apt_proxy_url` argument.
   The driver reports facts and the application names warnings (napalm-device-types 4.14):