Fixed: HTTP 415 error at operations endpoint when using OpenAPI extension

[bkoelman]

Before this PR, using Content-Type: application/vnd.api+json; ext="https://www.jsonapi.net/ext/openapi https://jsonapi.org/ext/atomic" would return HTTP 415 (Unsupported Media Type).
This happened because ASP.NET's own content negotiation middleware (which executes after ours) validates against the [Consumes] we inject, which didn't include the https://www.jsonapi.net/ext/openapi JSON:API extension.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.21%. Comparing base (e9b8f63) to head (023f710).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1971      +/-   ##
==========================================
+ Coverage   92.20%   92.21%   +0.01%     
==========================================
  Files         437      437              
  Lines       14924    14946      +22     
  Branches     2476     2476              
==========================================
+ Hits        13760    13782      +22     
  Misses        714      714              
  Partials      450      450              

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