add option to ignore missing fields in field_manager

[ekneg54]

this adds an option of ignoring missing source fields for some processors and tests for a bug that processing does not continue if a source_field is missing

[codecov-commenter]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (eb4db4b) 91.91% compared to head (9ccc64f) 92.03%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #457      +/-   ##
==========================================
+ Coverage   91.91%   92.03%   +0.11%     
==========================================
  Files         132      132              
  Lines        9476     9505      +29     
==========================================
+ Hits         8710     8748      +38     
+ Misses        766      757       -9     

Files	Coverage Δ
logprep/abc/processor.py	99.42% <100.00%> (+<0.01%)	⬆️
logprep/processor/amides/rule.py	100.00% <100.00%> (ø)
logprep/processor/calculator/rule.py	100.00% <100.00%> (ø)
logprep/processor/concatenator/processor.py	94.11% <100.00%> (+0.36%)	⬆️
logprep/processor/datetime_extractor/rule.py	100.00% <100.00%> (ø)
logprep/processor/dissector/processor.py	100.00% <100.00%> (ø)
logprep/processor/domain_label_extractor/rule.py	100.00% <100.00%> (ø)
logprep/processor/domain_resolver/rule.py	100.00% <100.00%> (ø)
logprep/processor/field_manager/processor.py	100.00% <100.00%> (ø)
logprep/processor/field_manager/rule.py	95.34% <100.00%> (+0.47%)	⬆️
... and 11 more

... and 1 file with indirect coverage changes

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