Make it possible to use `FilePublisher` without running a nameserver

[pnuu]

This is part of my slowly ongoing plan on using Trollflow2 in a container on operations. For our (FMI) use case it might be necessary to use direct port connections between different parts of the whole chain (Trollmoves Server -> Trollmoves Client -> segment gatherer -> Trollflow2 -> Georest -> Geoserver) without any of the steps using Posttroll nameserverss.

The default (below) usage will still require a Posttroll nameserver, and the publisher will default to use the one on `localhost'. By default the publish port is automatically selected

workers:
  - fun: !!python/object:trollflow2.plugins.FilePublisher {}

To use direct port communications without connecting to a nameserver:

workers:
  - fun: !!python/object:trollflow2.plugins.FilePublisher {port=40000, nameserver: null}

As with the default use, the messages will be published on the localhost in a port either given by the user or selected automatically by Posttroll, but with the feature added here the nameserver will not be needed if the port is known.

• [x] Tests added
• [x] Tests passed
• [x] Passes flake8 trollflow2
• [ ] Fully documented

[codecov[bot]]

Codecov Report

Merging #108 (88c84f8) into main (ffc9ac8) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
+ Coverage   93.95%   94.02%   +0.06%     
==========================================
  Files           9        9              
  Lines        1804     1824      +20     
==========================================
+ Hits         1695     1715      +20     
  Misses        109      109              

Flag	Coverage Δ
unittests	94.02% <100.00%> (+0.06%)	:arrow_up:

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Impacted Files	Coverage Δ
trollflow2/plugins/__init__.py	90.07% <100.00%> (+0.04%)	:arrow_up:
trollflow2/tests/test_trollflow2.py	99.34% <100.00%> (+0.01%)	:arrow_up:

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