Feature request: allow the user to edit the `systemd.journal-fields`

[hab25]

From man journalctl, DESCRIPTION section:

If one or more match arguments are passed, the output is filtered accordingly. A match is in the format "FIELD=VALUE", e.g. "_SYSTEMD_UNIT=httpd.service", referring to the components of a structured journal entry. See systemd.journal-fields(7) for a list of well-known fields.

N.b. even if journalctl-mode evolves such that it implements all systemd.journal-fields via transient infixes, systemd may evolve to provide more systemd.journal-fields; in that case this option would be useful in the timespan where journactl-mode hasn't caught up. Because at no point in the future can it be guaranteed that this situation will stop happening, this feature would never cease to be potentially useful.