Add OpenAPI 3 specification and source generation for REST API

[michael-valdron]

Please specify the area for this PR

index server

What does does this PR do / why we need it:

Replaces old OpenAPI 2.x swagger spec with an OpenAPI 3.x spec with a source generation script for the RESTful API endpoints of the devfile registry index server. Additionally, source for the endpoints have been refactored to connect with generated endpoint definitions.

Type schemas used by the index server source, such as the index schema (but still referenced by the index generator), are now also generated from the spec. All generated source files have the pattern *.gen.go.

Which issue(s) this PR fixes:

Fixes #?

fixes devfile/api#972

PR acceptance criteria:

• Test Coverage
  • Are your changes sufficiently tested, and are any applicable test cases added or updated to cover your changes?

Documentation (WIP)

• Does the REST API doc need to be updated with your changes?
• Does the registry library doc need to be updated with your changes?

How to test changes / Special notes to the reviewer:

[codecov]

Codecov Report

Patch coverage: 15.33% and project coverage change: -9.77 ⚠️

Comparison is base (bdfb830) 45.69% compared to head (10a0c1d) 35.93%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
- Coverage   45.69%   35.93%   -9.77%     
==========================================
  Files           6        7       +1     
  Lines        1057     1347     +290     
==========================================
+ Hits          483      484       +1     
- Misses        533      816     +283     
- Partials       41       47       +6     

Impacted Files	Coverage Δ
index/server/pkg/server/index.go	0.00% <0.00%> (-9.03%)	⬇️
index/server/pkg/server/endpoint.gen.go	2.69% <2.69%> (ø)
index/server/pkg/server/endpoint.go	40.90% <52.77%> (-1.95%)	⬇️

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[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: maysunfaisal, michael-valdron

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