Add environment variables to override public transit plugins
In relation to #256 I think it might be useful to manually "force" a certain plugin for transit routing. Since right now one would have to write a custom services file and set MAPS_SERVICE, which is a bit tricky, you have to specify bounding boxes. And it also is probably hard to use with Flatpak, as it needs to be able to read a file.
Something like:
TRANSIT_PLUGIN=NameOfPlugin
would use a class NameOfPlugin from transitplugins/nameOfPlugin.js to use to test out a newly developed third-party plugin.
And also to allow testing out existing instances of OpenTripPlanner (to use with TRANSIT_PLUGIN=OpenTripPlanner) to manually set a base URL and (optionally) specify if the instance only has transit data (e.g. to use GraphHopper for the walking routes).
OTP_BASE_URL=https://example.com/otp
OTP_TRANSIT_ONLY=true
something along those lines