refactor: decouple toDeckConfig/OnUpdate from controller, and expose as libraries

[mflendrich]

This PR:

• introduces the package pkg/deckgen that exposes ToDeckContent() acting as a converter from KongState to file.Content
  • introduces dependency injection for auxiliary info required by ToDeckContent() (as opposed to ToDeckContent() querying data sources for info directly)
• introduces the package pkg/sendconfig that connects to Kong Admin API in order to configure it according to file.Content
• straightens out the logic of SHA diffing

Why we need this change: in order to expose the existing (KongState -> file.Content and file.Content -> Kong Admin API) operations as a library, to be used by the prototype port of KIC to kubebuilder.

Please review commit-by-commit. None of the commits (except for the modified SHA diffing) should change existing behavior. I recommend merging without squashing either - because individual moves are much easier to comprehend in individual commits.

Testing plan:

• Unit tests ran, coverage not changed here. Writing unit tests (for the now exposed functions that are now much easier to unit test) out of scope of this PR.
• Tested the DB-less mode to the extent of the present E2E test.
• DB mode not tested automatically. In parallel with the review, the author will perform the test manually (the test will include manual validation of the SHA logic).

[codecov]

Codecov Report

Merging #1029 (90f818d) into next (b3be780) will increase coverage by 2.41%.
The diff coverage is 16.25%.

@@            Coverage Diff             @@
##             next    #1029      +/-   ##
==========================================
+ Coverage   51.42%   53.84%   +2.41%     
==========================================
  Files          33       33              
  Lines        3327     3172     -155     
==========================================
- Hits         1711     1708       -3     
+ Misses       1485     1332     -153     
- Partials      131      132       +1     

Impacted Files	Coverage Δ
cli/ingress-controller/main.go	0.23% <0.00%> (ø)
pkg/deckgen/generate.go	0.00% <0.00%> (ø)
pkg/util/plugin_schema_helper.go	0.00% <0.00%> (ø)
pkg/sendconfig/sendconfig.go	20.00% <20.00%> (ø)
pkg/deckgen/deckgen.go	39.18% <39.18%> (ø)
pkg/parser/parser.go	83.92% <0.00%> (-1.34%)	⬇️
internal/ingress/controller/event_handler.go
... and 1 more

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[rainest]

I'll need to rework #991 around this but have had difficulty fitting it around the existing monolithic functions, so agreed that breaking this functionality out will make it easier to add new components around them. Thanks for the refactor 👍

[shaneutt]

Looks good. I have no blocking comments, but I do have a couple smaller things related to godoc and comments. 👍

[shaneutt]

I know this was just imported from existing code, but can we improve this with an issue link or any other breadcrumbs for the reader?