CIF-1300 - Implement mock GraphQL HTTP endpoint for the components library pages

[cjelger]

Description

In order to have a self-contained content package for the components library pages, we develop a mock GraphQL HTTP endpoint that would contain sample JSON responses for all the CIF components of the library pages.

This PR contains an AEM Servlet that implements that GraphQL server. It uses the graphql-java to implement a real GraphQL server using the Magento 2.3.4 GraphQL schema. The JSON responses are mocked, and should be refined/adapted when we will start developing the library pages.

How Has This Been Tested?

The Servlet has been tested with all the server-side CIF Components that will be potentially used in the library pages. Each test uses the corresponding Sling Model and data retriever, so we exactly test all the GraphQL queries issued by the components.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes and the overall coverage did not decrease.
• All unit tests pass on CircleCi.
• I ran all tests locally and they pass.

[codecov]

Codecov Report

Merging #224 into master will increase coverage by 0.67%.
The diff coverage is 90.69%.

@@             Coverage Diff              @@
##             master     #224      +/-   ##
============================================
+ Coverage     62.32%   63.00%   +0.67%     
- Complexity      728      758      +30     
============================================
  Files           171      172       +1     
  Lines          5240     5368     +128     
  Branches        820      838      +18     
============================================
+ Hits           3266     3382     +116     
- Misses         1862     1869       +7     
- Partials        112      117       +5     

Flag	Coverage Δ	Complexity Δ
#jest	40.21% <ø> (ø)	0.00 <ø> (ø)
#karma	94.88% <ø> (ø)	0.00 <ø> (ø)
#unittests	85.16% <90.69%> (+0.34%)	758.00 <30.00> (+30.00)

Impacted Files	Coverage Δ	Complexity Δ
...models/v1/relatedproducts/RelatedProductsImpl.java	86.66% <0.00%> (-2.23%)	15.00 <0.00> (ø)
...ommerce/core/examples/servlets/GraphqlServlet.java	91.40% <91.40%> (ø)	30.00 <30.00> (?)

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