Allow SIRI's StopPointRef to refer to a NeTEx scheduled stop point

[leonardehrenfried]

Summary

Even though the various documents on SIRI and NeTEx are quite unclear, even contradictory what exactly an EstimatedCall's StopPointRef should refer to, all the experts agree that it a scheduled stop point is probably a better target.

We discussed this at length at https://public.3.basecamp.com/p/TcEEP5WrNZJPBxrJU9GAjint

For this reason, this adds support for SIRI to refer to scheduled stop point.

Issue

#6340

Unit tests

I added some for the EntityResolver and the TimetableRepository.

Documentation

I didn't want to duplicate the documentation for this in several places so I opted to documented it in the innermost layer (TimetableRepository) and link to it in other places.

Bumping the serialization version id

✔️

cc @rcavaliere

[codecov]

Codecov Report

Attention: Patch coverage is 88.09524% with 5 lines in your changes missing coverage. Please review.

Project coverage is 69.74%. Comparing base (1030408) to head (59bf199).

Files with missing lines	Patch %	Lines
...anner/netex/loader/parser/NetexDocumentParser.java	78.57%	1 Missing and 2 partials ⚠️
...lanner/netex/loader/parser/ServiceFrameParser.java	0.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #6397   +/-   ##
==========================================
  Coverage      69.73%   69.74%           
- Complexity     18024    18033    +9     
==========================================
  Files           2057     2057           
  Lines          76970    76994   +24     
  Branches        7846     7846           
==========================================
+ Hits           53678    53702   +24     
+ Misses         20545    20544    -1     
- Partials        2747     2748    +1     

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