Add mapper for GTFS GraphQL real time state

[leonardehrenfried]

Summary

After #6045 was merged I realised that we should have a proper mapper for the realtime state since this forces you to correctly deal with all enum types.

Case in point, the current code would break with the real time state DELETED. I've mapped it to CANCELLED.

Issue

None.

Unit tests

None.

[codecov]

Codecov Report

Attention: Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.

Project coverage is 69.84%. Comparing base (2cfbd7b) to head (d49e5fa).
Report is 26 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...planner/apis/gtfs/mapping/RealtimeStateMapper.java	37.50%	4 Missing and 1 partial ⚠️
...ipplanner/apis/gtfs/datafetchers/StoptimeImpl.java	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             dev-2.x    #6099   +/-   ##
==========================================
  Coverage      69.83%   69.84%           
- Complexity     17461    17466    +5     
==========================================
  Files           1975     1976    +1     
  Lines          74624    74632    +8     
  Branches        7638     7638           
==========================================
+ Hits           52115    52126   +11     
+ Misses         19858    19855    -3     
  Partials        2651     2651           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.