MINIFICPP-1526 Add ConsumeJournald to consume systemd journal messages

[szaszm]

• libsystemd might be problematic license-wise. I might have to work around it with dlopen.
• Still figuring out what would be the best way to test this.
• Tests are failing, work in progress, but submitting the PR anyway to see the reaction and gather opinions.

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[fgerlits]

just some minor comments

[lordgamez]

When I try testing the processor manually on my local machine it fails with the following error:

[2021-05-13 14:00:02.434] [org::apache::nifi::minifi::core::ProcessGroup] [error] Failed to start processor d9606075-a35a-4509-b705-13e793e1b82b (ConsumeJournald): sd_journal_get_cursor: Cannot assign requested address

I could only find a similar issue but I did not find a solution for it, so it still needs to be investigated.