Add endpoint to download Transmodel GraphQL Schema

[t2gran]

Summary

This PR add a read-only endpoint (HTTP GET) for fetching the Transmodel GraphQL Schema. The schema can be fetched today using the GraphQL introspection, but this is a bit technical and the result need to be translated into SDL (if needed).

We will use this to generate the input controls for the trip search in the OTP Debug UI. It can also be used for "human introspection".

Issue

🟥 No issue, but this is part of making a generic input control for the Debug UI.

Unit tests

🟥 This is just one line of code with logic - only using library code. The schema is for

Documentation

✅ The usage is documented with a comment included at the top of the schema file.
🟥 The endpoint is not documented outside the code - this should be sufficient since it is not intended
for production/integration - only for the convenience of developers.

Changelog

🟥 Not a production feature or relevant for Product Owners

Bumping the serialization version id

🟥 Not needed

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 69.81%. Comparing base (e0422cd) to head (f765199).
Report is 33 commits behind head on dev-2.x.

Files with missing lines	Patch %	Lines
...opentripplanner/apis/transmodel/TransmodelAPI.java	0.00%	2 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6330      +/-   ##
=============================================
+ Coverage      69.79%   69.81%   +0.01%     
- Complexity     17797    17823      +26     
=============================================
  Files           2019     2020       +1     
  Lines          76119    76269     +150     
  Branches        7786     7806      +20     
=============================================
+ Hits           53129    53246     +117     
- Misses         20284    20306      +22     
- Partials        2706     2717      +11     

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