feat(diag) expose fallback objects

[rainest]

What this PR does / why we need it:

The bit requested in #6101 (comment)

Which issue this PR fixes:

Fix #5932

Special notes for your reviewer:

Nothing re #4473 as requested, and no tests at present, nor a fix for the bit that's broken per #5932 (comment), but putting something up given requested timeline.

Unfortunately didn't see the design edit on #5932, but this at least provides the piping needed to ship stuff to the diag server to try and reshape it into that.

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• the CHANGELOG.md release notes have been updated to reflect any significant (and particularly user-facing) changes introduced by this PR

[codecov]

Codecov Report

Attention: Patch coverage is 55.69620% with 70 lines in your changes missing coverage. Please review.

Project coverage is 69.7%. Comparing base (363a891) to head (981e3a1).
Report is 4 commits behind head on main.

❗ Current head 981e3a1 differs from pull request most recent head f1e2352

Please upload reports for the commit f1e2352 to get more accurate results.

Files	Patch %	Lines
internal/diagnostics/mapping.go	0.0%	30 Missing ⚠️
internal/dataplane/kong_client.go	53.0%	20 Missing and 3 partials ⚠️
internal/dataplane/fallback/fallback.go	47.6%	11 Missing ⚠️
internal/diagnostics/server.go	82.3%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #6159     +/-   ##
=======================================
- Coverage   73.8%   69.7%   -4.2%     
=======================================
  Files        198     200      +2     
  Lines      19798   19885     +87     
=======================================
- Hits       14619   13865    -754     
- Misses      4197    5031    +834     
- Partials     982     989      +7     

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[czeslavo]

We should make sure that no fallback objects are reported after we successfully push non-fallback configuration.

[rainest]

Alright, so discarding all the hash stuff and separate problem endpoint,

$ curl -sv localhost:10256/debug/config/fallback | jq
{
  "objects": [
    {
      "resource": "configuration.konghq.com/KongPlugin",
      "namespace": "default",
      "name": "key-auth",
      "id": "52fd9412-56c9-4d09-94cf-55860e34e080",
      "status": "excluded",
      "cause": "configuration.konghq.com/KongPlugin:default/key-auth"
    },
    {
      "resource": "gateway.networking.k8s.io/HTTPRoute",
      "namespace": "default",
      "name": "httproute-testing2",
      "id": "dd336bc8-1978-4185-a819-293ac666f87b",
      "status": "excluded",
      "cause": "configuration.konghq.com/KongPlugin:default/key-auth"
    }
  ]
}

I'm not sure if there are currently or would ever be cases where we'd have a mix of excluded and backfilled objects--the code suggests we can only have one or the other for the current fallback state.

[czeslavo]

As a result of the review, I prepared a suggested changes set, but we agreed with @pmalek to merge it to feat/diag-fallback-more already to save time and review it as a whole in this PR.

[programmer04]

overall LGMT