u-u: implement plugin system with `postrun()` functionality

[mvo5]

[opening as draft, this needs a bit more work, I think the right way is to add a namedtuple or a dataclass for class UnattendedUpgradeResult

This commit implements a new plugin system. Plugins are python classes that start with UnattendedUpgradesPlugin and that are locates in one of the following directories:

/etc/unattended-upgrades/plugins/
/usr/share/unattended-upgrades/plugins

Plugins are also searched in the UNATTENDED_UPGRADES_PLUGIN_PATH environment path and in any of the directories in the apt configration: Unattended-Upgrade::Dirs::Plugins::.

The postrun() function is called with a single python dictionary as the argument. This dictionary contains the following elements:

• "plugin-api": the API version as string of the form "1.0"
• "hostname": The hostname of the machine that run u-u.
• "success": A boolean that indicates if the run was successful
• "result-str": A string with a human readable (and translated) status message
• "packages-upgraded": A list of packages that got upgraded.
• "packages-kept-back": A list of packages kept back.
• "packages-kept-installed": A list of packages not auto-removed.
• "reboot-required": Indicates a reboot is required.
• "log-dpkg": The full dpkg log output.
• "log-unattended-upgrades": The full unattended-upgrades log.

This new code can be tested with:

$ sudo PYTHONPATH=. UNATTENDED_UPGRADES_PLUGIN_PATH=./examples/plugins/ ./unattended-upgrade

[papamoose]

After changing some keys in my apprise plugin this still seems to work as expected. I've deployed this PR where I'm able for testing.

[mvo5]

After changing some keys in my apprise plugin this still seems to work as expected. I've deployed this PR where I'm able for testing.

Thanks for the testing and sorry for these slightly incompatibilities in this final version, once this is merged/released that will (obviously) not change again :)

[mvo5]

I tweaked the documentation for the new feature a little bit and I think it's ready now . If someone could double check for clarity/spelling/typos that would be appreciated! I want to merge this soon(ish).

[papamoose]

After changing some keys in my apprise plugin this still seems to work as expected. I've deployed this PR where I'm able for testing.

Looks good so far. All systems are reporting in as they should.