Support for existsBy projection

[micw]

Fixes #93

[codecov-io]

Codecov Report

Merging #94 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master      #94     +/-   ##
===========================================
+ Coverage     40.58%   40.68%   +0.1%     
- Complexity      336      337      +1     
===========================================
  Files            64       64             
  Lines          1752     1755      +3     
  Branches        341      343      +2     
===========================================
+ Hits            711      714      +3     
  Misses          936      936             
  Partials        105      105

Impacted Files	Coverage Δ	Complexity Δ
...namodb/repository/query/PartTreeDynamoDBQuery.java	80.95% <100%> (+0.95%)	9 <2> (+1)	⬆️
...namodb/repository/query/AbstractDynamoDBQuery.java	21.49% <100%> (+1.49%)	7 <0> (ø)	⬇️

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[derjust]

Thank you - especially the addition of the unit tests!