Expose debug endpoints allowing inspection of the fallback configuration

[czeslavo]

Make sure that /debug/config endpoints allowing inspection of the generated fallback configuration exist.

Proposed solution

/debug/config/problems endpoint should return responses including the following data.

When objects were only excluded

{
    "brokenObjects": [
        {"kind": "KongPlugin", "name": "key-auth", "namespace": "default"}, 
        {"kind": "HTTPRoute", "name": "route-b", "namespace": "default"}
    ],
    "excludedObjects": [
        {"kind": "KongPlugin", "name": "key-auth", "namespace": "default"}, 
        {"kind": "HTTPRoute", "name": "route-b", "namespace": "default"}
    ]
    "backfilledObjects": []
}

When objects were excluded and then backfilled

{
    "brokenObjects": [
        {"kind": "KongPlugin", "name": "key-auth", "namespace": "default"}, 
        {"kind": "HTTPRoute", "name": "route-b", "namespace": "default"}
    ]
    "excludedObjects": [
        {"kind": "KongPlugin", "name": "key-auth", "namespace": "default"}, 
        {"kind": "HTTPRoute", "name": "route-b", "namespace": "default"}
    ],
    "backfilledObjects": [
        {"kind": "HTTPRoute", "name": "route-b", "namespace": "default"}
    ]
}

When a regular config push succeeded (no fallback configuration was involved)

{
    "brokenObjects": [],
    "excludedObjects": [],
    "backfilledObjects": []
}

Acceptance criteria:

• Users can list all broken objects (the ones that were rejected by the Gateway)
• Users can list all excluded objects (the ones that were excluded in the fallback configuration)
• Users can list all backfilled objects (the ones that were backfilled in the fallback configuration)

[czeslavo]

@rainest Let's see if we can solve #4473 along with this.

[czeslavo]

@rainest I noticed that /debug/config/problems always returns an empty slice of broken objects and no hash, even if the fallback configuration generation is triggered. This can be verified following the steps from the guide. Not creating a separate issue as I believe we should handle it under this one.

I updated the issue with the proposed content of the responses.