suggestion: fallback config diagnostics server

[czeslavo]

• Extracts GeneratedCacheMetadataCollector for easier metadata collection (deduplication of excluded/backfilled objects was needed) and isolation
• Adds unit tests for the diagnostics server and fallback config generator
• Changes the response schema to include both excluded and backfilled objects at the same time (as that is a valid scenario: an object can be first excluded, then backfilled or not depending on the last valid cache state)
• Ensures we push diagnostics only if they're enabled (c.diagnostic.FallbackCacheMetadata is not nil)
• Extracts diagnostics server response schema types to a separate file for readability
• Extracts config dump and fallback diagnostics retrieval handling methods in the server
• Added status field to the response to clearly indicate whether the fallback config generation was triggered or not
• Fixes cosmetic issues

Sample responses from manual testing:

excluded + backfilled

{
  "status": "triggered",
  "brokenObjects": [
    {
      "group": "configuration.konghq.com",
      "kind": "KongPlugin",
      "namespace": "default",
      "name": "rate-limit-consumer",
      "id": "0a823310-7b01-4e8e-b85e-1ebb160ddc3e"
    }
  ],
  "excludedObjects": [
    {
      "group": "configuration.konghq.com",
      "kind": "KongPlugin",
      "version": "v1",
      "namespace": "default",
      "name": "rate-limit-consumer",
      "id": "0a823310-7b01-4e8e-b85e-1ebb160ddc3e",
      "causingObjects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    },
    {
      "group": "gateway.networking.k8s.io",
      "kind": "HTTPRoute",
      "version": "v1",
      "namespace": "default",
      "name": "route-b",
      "id": "21c65967-909a-4a18-b875-c8d5652ee538",
      "causingObjects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    }
  ],
  "backfilledObjects": [
    {
      "group": "gateway.networking.k8s.io",
      "kind": "HTTPRoute",
      "version": "v1",
      "namespace": "default",
      "name": "route-b",
      "id": "21c65967-909a-4a18-b875-c8d5652ee538",
      "causingObjects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    },
    {
      "group": "configuration.konghq.com",
      "kind": "KongPlugin",
      "version": "v1",
      "namespace": "default",
      "name": "rate-limit-consumer",
      "id": "0a823310-7b01-4e8e-b85e-1ebb160ddc3e",
      "causingObjects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    },
    {
      "group": "configuration.konghq.com",
      "kind": "KongConsumer",
      "version": "v1",
      "namespace": "default",
      "name": "bob",
      "id": "bd24d998-2b37-46b4-896e-634c910226d6",
      "causingObjects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    }
  ]
}

only excluded

{
  "status": "triggered",
  "brokenObjects": [
    "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
  ],
  "excludedObjects": [
    {
      "resource": "configuration.konghq.com/v1, Kind=KongPlugin",
      "namespace": "default",
      "name": "rate-limit-consumer",
      "id": "0a823310-7b01-4e8e-b85e-1ebb160ddc3e",
      "causing_objects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    },
    {
      "resource": "gateway.networking.k8s.io/v1, Kind=HTTPRoute",
      "namespace": "default",
      "name": "route-b",
      "id": "21c65967-909a-4a18-b875-c8d5652ee538",
      "causing_objects": [
        "configuration.konghq.com/KongPlugin:default/rate-limit-consumer"
      ]
    }
  ]
}

no fallback triggered

{
  "status": "not_triggered"
}