Tighten GraphQL definition of environment type

[sersorrel]

This is already assumed to be the case by the frontend.

Also document how to generate a GraphQL .schema file, which can be used by the frontend for type-checking.

[mjkw31]

LGTM.

[codecov]

Codecov Report

Attention: 27 lines in your changes are missing coverage. Please review.

Comparison is base (16e49df) 65.29% compared to head (8ce9ddb) 65.28%.
Report is 6 commits behind head on develop.

Files	Patch %	Lines
softpack_core/schemas/environment.py	29.03%	22 Missing ⚠️
softpack_core/artifacts.py	76.92%	3 Missing ⚠️
softpack_core/schemas/groups.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop      #43      +/-   ##
===========================================
- Coverage    65.29%   65.28%   -0.01%     
===========================================
  Files           16       17       +1     
  Lines          775      818      +43     
  Branches       131      141      +10     
===========================================
+ Hits           506      534      +28     
- Misses         262      277      +15     
  Partials         7        7              

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