Configuration of additional HTTP headers for GTFS-RT updaters

[leonardehrenfried]

Summary

This adds configuration parameters so that HTTP headers can be configured for GTFS-RT updaters for trip updates, vehicle positions and alerts.

I've also slightly reworked how the example JSON snippets are formatted.

Issue

Closes #2544.

Unit tests

Added for MapUtils.combine.

Documentation

Autogenerated.

[codecov]

Codecov Report

Base: 61.15% // Head: 61.41% // Increases project coverage by +0.26% 🎉

Coverage data is based on head (56edbb3) compared to base (5eb33c1).
Patch coverage: 79.45% of modified lines in pull request are covered.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #4684      +/-   ##
=============================================
+ Coverage      61.15%   61.41%   +0.26%     
  Complexity     12437    12437              
=============================================
  Files           1597     1595       -2     
  Lines          63997    63756     -241     
  Branches        6982     6910      -72     
=============================================
+ Hits           39136    39157      +21     
+ Misses         22648    22384     -264     
- Partials        2213     2215       +2     

Impacted Files	Coverage Δ
...anner/updater/alert/GtfsRealtimeAlertsUpdater.java	0.00% <0.00%> (ø)
...updater/trip/GtfsRealtimeHttpTripUpdateSource.java	0.00% <0.00%> (ø)
...ehicle_position/PollingVehiclePositionUpdater.java	0.00% <0.00%> (ø)
...ner/updater/trip/PollingTripUpdaterParameters.java	21.42% <50.00%> (-21.43%)	⬇️
...ter/alert/GtfsRealtimeAlertsUpdaterParameters.java	75.00% <66.66%> (+17.85%)	⬆️
...outerconfig/updaters/PollingTripUpdaterConfig.java	80.00% <90.00%> (+4.00%)	⬆️
...opentripplanner/framework/collection/MapUtils.java	90.90% <100.00%> (+3.40%)	⬆️
...standalone/config/routerconfig/UpdatersConfig.java	59.49% <100.00%> (+2.16%)	⬆️
...nfig/updaters/GtfsRealtimeAlertsUpdaterConfig.java	95.23% <100.00%> (+9.52%)	⬆️
...config/updaters/VehiclePositionsUpdaterConfig.java	95.45% <100.00%> (+1.33%)	⬆️
... and 12 more

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