Add HTTP headers to GraphiQL, upgrade version

[leonardehrenfried]

Summary

One of my deployments is using rate limiting and API keys so I would like to be able to set headers in GraphiQL, which this PR adds.

While I was there I also upgraded GraphiQL to the latest version.

cc @fahrplaner

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.80%. Comparing base (71bb5fa) to head (ed16268).
Report is 11 commits behind head on dev-2.x.

Additional details and impacted files

@@              Coverage Diff              @@
##             dev-2.x    #6380      +/-   ##
=============================================
- Coverage      69.80%   69.80%   -0.01%     
+ Complexity     17942    17941       -1     
=============================================
  Files           2046     2046              
  Lines          76671    76671              
  Branches        7830     7830              
=============================================
- Hits           53520    53518       -2     
- Misses         20408    20411       +3     
+ Partials        2743     2742       -1     

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[optionsome]

It's a bit unfortunate that the UI doesn't really give hints in what format the headers/variables should be provided in (apart from the error messages).