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 a scheduled stop point is a valid target (as an alternative to the quay).

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

Closes #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 document 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 86.04651% with 6 lines in your changes missing coverage. Please review.

Project coverage is 69.87%. Comparing base (448ad8f) to head (b69d085).
Report is 252 commits behind head on dev-2.x.

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 ⚠️
...planner/transit/service/DefaultTransitService.java	0.00%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6397      +/-   ##
=============================================
+ Coverage      69.72%   69.87%   +0.14%     
- Complexity     18016    18126     +110     
=============================================
  Files           2057     2069      +12     
  Lines          76967    77262     +295     
  Branches        7844     7844              
=============================================
+ Hits           53666    53984     +318     
+ Misses         20550    20524      -26     
- Partials        2751     2754       +3     

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