fix: handle flattened errors related to whole entities

[czeslavo]

What this PR does / why we need it:

When an error in the flattened errors response is related to a whole entity (its type=entity), we can generate a more descriptive key for this kind of problem by building it from the entity type and name instead of accidentally using an empty string.

{
  "fields": {},
  "flattened_errors": [
    {
      "errors": [
        {
          "type": "field",
          "message": "value must be null",  
          "field": "path"
        },
        {
          "type": "entity",  <-- Make use of that to determine if the error is associated with a single field or a whole entity.
          "message": "failed conditional validation given value of field protocol"
        }
      ],
      "entity": {
        "host": "konghq.com",
        "name": "test-service",
        "port": 80,
        "path": "/test",
        "tags": [
          "k8s-name:test-service",
          "k8s-namespace:default",
          "k8s-kind:Service",
          "k8s-uid:a3b8afcc-9f19-42e4-aa8f-5866168c2ad3",
          "k8s-group:",
          "k8s-version:v1"
        ],
        "protocol": "grpc"
      },
      "entity_type": "service",  <-- Use the entity type to build the problems map keys.
      "entity_name": "test-service", <-- Use the entity name to build the problems map keys.
      "entity_tags": [
        "k8s-name:test-service",
        "k8s-namespace:default",
        "k8s-kind:Service",
        "k8s-uid:a3b8afcc-9f19-42e4-aa8f-5866168c2ad3",
        "k8s-group:",
        "k8s-version:v1"
      ]
    }
  ],
  "name": "invalid declarative configuration",
  "message": "declarative config is invalid: {}",
  "code": 14
}

Which issue this PR fixes:

Follow up of #4802 (comment).

[codecov]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (7cd5218) 77.9% compared to head (aecdceb) 75.7%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4813     +/-   ##
=======================================
- Coverage   77.9%   75.7%   -2.2%     
=======================================
  Files        163     163             
  Lines      18510   18515      +5     
=======================================
- Hits       14422   14019    -403     
- Misses      3277    3699    +422     
+ Partials     811     797     -14     

Files	Coverage Δ
...al/dataplane/sendconfig/inmemory_error_handling.go	63.8% <100.0%> (+2.3%)	⬆️
internal/dataplane/sendconfig/sendconfig.go	82.4% <75.0%> (ø)

... and 14 files with indirect coverage changes

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[pmalek]