feat: apt's proxy on a Proxmox VE node (NetOrk/netork#1397)

The node is Debian: AptProxyMixin gives it get_package_proxy(),
get_package_proxy_report(), check_package_proxy() and the fix
configure_package_proxy(), as on any Debian host. Logged in as root, the
transports are the exec path; the file is written without an exit inside
_rc's braces, which would end the shell before the status (see #30).

Requires napalm-device-types 4.22.0; version 0.7.0.

CHANGELOG.md

@@ -8,6 +8,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- apt's proxy (napalm-device-types' `AptProxyMixin`, NetOrk/netork#1397): `get_package_proxy()`, `get_package_proxy_report()`, `check_package_proxy(url, expect)` and the fix `configure_package_proxy(url)`, as on any Debian host. The node is logged in as root, so the file is written and other proxy lines are commented out over the exec path. Requires napalm-device-types 4.22.0; version 0.7.0.
 - `get_service_packages()` (napalm-device-types' `ServicePackagesMixin`, NetOrk/netork#1236): which installed packages each systemd unit of the node is made of — the one shipping its unit file and the one shipping its `ExecStart` binary. Read-only, over the exec path. Requires napalm-device-types 4.20.0; version 0.6.0.
 
 ### Security