feat: AptProxyMixin, what apt fetches through and pointing it at a proxy (NetOrk/netork#1397)

apt reads apt.conf.d in order and apt.conf last; the last value wins. The
Debian installer writes its proxy into apt.conf, Ubuntu's into
90curtin-aptproxy: reading 00proxy alone missed both, and a fix that wrote
00proxy and read it back reported success while apt fetched elsewhere.

- get_package_proxy_report(): apt-config dump's proxy, per-host overrides,
  auto-detect, every line of apt's files that sets it (in reading order,
  editable or inside a block), the http and https sources.
- check_package_proxy(url, expect): a fetch with apt-helper, past any proxy;
  an HTTP error counts as an answer.
- configure_package_proxy(url): write 00proxy, comment out other proxy
  lines, read apt's proxy back, fetch one InRelease through it.
- same_proxy_url(): scheme, host and port, a trailing slash aside.

A driver supplies _run_apt_command, _run_apt_root_command and
_write_apt_file. Version 4.22.0.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "napalm-device-types"
-version = "4.21.0"
+version = "4.22.0"
 description = "Abstract device-type base classes for NAPALM drivers"
 readme = "README.md"
 requires-python = ">=3.10"