Add support for XML stream reader

[vpaturet]

Add support for reading a NeTEx document through an XML stream reader (StAX).
This gives the possibility to filter out XML elements through a custom XML stream reader to reduce memory footprint.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 26.66667% with 11 lines in your changes missing coverage. Please review.

Please upload report for BASE (master@ab83186). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
src/main/java/org/entur/netex/NetexParser.java	33.33%	7 Missing and 1 partial ⚠️
...in/java/org/entur/netex/loader/NetexXmlParser.java	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #243   +/-   ##
=========================================
  Coverage          ?   88.43%           
  Complexity        ?      234           
=========================================
  Files             ?       16           
  Lines             ?      813           
  Branches          ?       60           
=========================================
  Hits              ?      719           
  Misses            ?       66           
  Partials          ?       28           

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