fix: Expose ExplainEnum in the GQL schema

[orpheuslummis]

Relevant issue(s)

Resolves #1202

Description

Adds ExplainEnum to the GQL schema.
It fixes the introspection query error with Altair and GraphiQL.

Adds a test for the enum exist in resulting response schema . More general test cases should be preferred to this, such as general GraphQL schema validation.

(Discussion point?: It is a draft PR because I'm unsure whether to also change the schema integration test suite to handle graphql schema validation as a way to test 'large' schema responses from 'large' introspection queries.
Resolution: use assertContains to test the specific property, later introduce schema validation testing.)

Tasks

• I made sure the code is well commented, particularly hard-to-understand areas.
• I made sure the repository-held documentation is changed accordingly.
• I made sure the pull request title adheres to the conventional commit style (the subset used in the project can be found in tools/configs/chglog/config.yml).
• I made sure to discuss its limitations such as threats to validity, vulnerability to mistake and misuse, robustness to invalidation of assumptions, resource requirements, ...

How has this been tested?

Specify the platform(s) on which this was tested:

• MacOS

[codecov]

Codecov Report

Merging #1204 (b07f650) into develop (0077635) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

❗ Current head b07f650 differs from pull request most recent head 7fdd911. Consider uploading reports for the commit 7fdd911 to get more accurate results

@@             Coverage Diff             @@
##           develop    #1204      +/-   ##
===========================================
- Coverage    68.70%   68.62%   -0.09%     
===========================================
  Files          181      181              
  Lines        17123    17125       +2     
===========================================
- Hits         11765    11752      -13     
- Misses        4407     4417      +10     
- Partials       951      956       +5     

Impacted Files	Coverage Δ
request/graphql/schema/types/types.go	100.00% <ø> (ø)
request/graphql/schema/manager.go	97.36% <100.00%> (+0.04%)	⬆️

... and 5 files with indirect coverage changes

[AndrewSisley]

Thanks Orpheus for find and sorting this! Prod code looks good, and thanks for adding the test - I do think that query needs to be trimmed down a bit before merge though.

[AndrewSisley]

Thanks Orpheus!