Allow filtering systemd unit in the journald input

[camdencheek]

The journald_input operator currently does not allow the user to filter to certain streams. Often, users are only interested in logs from a specific systemd unit. We can filter after the fact with a filter operator, but that still requires ingesting and parsing all the journald logs even if we're only interested in a small fraction of them. The journalctl executable we use supports the --unit <systemd_unit> parameter, which will allow us to filter before the logs ever make it to stanza.

In addition to this, we may want to add the option to specify arbitrary additional arguments since journalctl provides many filtering, format, and other options. For example, users can specify which fields they want included, specify a minimum priority level, or include only logs from the latest boot.