Please make it possible to read from the Systemd journal.

Currently, the only way to get logging information out of Systemd services is to port the information to (r)syslog or use something like [Datadog's Vector product](https://vector.dev/blog/datadog-acquisition/). However, it would be great if we could reference Systemd journal just like we do with log files.

I propose that you add a new "logs" section named, `systemd`, as an addition to the existing `files` and `windows_events` sections.

Start by reviewing how the existing `files` and `windows_events` log sections are represented and processed. Define the scope of the proposed `systemd` logs section, including how it should read the Systemd journal and behave like file-based logs; done when the agent can collect and export those entries.

Written by the indexing model from the issue text.