fix: correctly fill plugins with arbitrary map fields

Kong plugin schemas support different type of fields,
including maps. Moreover, these can either be arbitrary
maps, or maps with their own subschema.

go-kong already supports well the second case, but it
falls short with arbitrary maps. An example of arbitrary
map field is the `custom_fields_by_lua` from the `http-log`
plugin, which is defined as follows:

```
{
  "custom_fields_by_lua": {
    "keys": {
      "len_min": 1,
      "type": "string"
    },
    "type": "map",
    "values": {
      "len_min": 1,
      "type": "string"
    }
  }
}
```

This commit makes sure that go-kong is able to correctly
handle such schemas as well.

kong/plugin_service_test.go

@@ -607,6 +607,38 @@ func TestFillPluginDefaults(T *testing.T) {
 				Enabled:   Bool(false),
 			},
 		},
+		{
+			name: "nested config with arbitrary map field",
+			plugin: &Plugin{
+				Name: String("http-log"),
+				Config: Configuration{
+					"custom_fields_by_lua": map[string]interface{}{
+						"foo": "bar",
+					},
+				},
+				Enabled:   Bool(false),
+				Protocols: []*string{String("grpc"), String("grpcs")},
+			},
+			expected: &Plugin{
+				Name: String("http-log"),
+				Config: Configuration{
+					"content_type": string("application/json"),
+					"custom_fields_by_lua": map[string]interface{}{
+						"foo": "bar",
+					},
+					"flush_timeout": float64(2),
+					"headers":       nil,
+					"http_endpoint": nil,
+					"keepalive":     float64(60000),
+					"method":        string("POST"),
+					"queue_size":    float64(1),
+					"retry_count":   float64(10),
+					"timeout":       float64(10000),
+				},
+				Enabled:   Bool(false),
+				Protocols: []*string{String("grpc"), String("grpcs")},
+			},
+		},
 	}
 
 	for _, tc := range tests {

kong/utils.go

@@ -217,12 +217,24 @@ func fillConfigRecord(schema gjson.Result, config Configuration) Configuration {
 		}
 
 		// check if key is already set in the config
-		if _, ok := config[fname]; ok {
+		if v, ok := config[fname]; ok {
 			// the field is set. If it's not a map, then
 			// the field is fully set. If it's a map, we
 			// need to make sure that all fields are properly
 			// filled.
-			if _, ok := config[fname].(map[string]interface{}); !ok {
+			//
+			// some fields are defined as arbitrary maps,
+			// containing a 'keys' and 'values' subfields.
+			// in this case, the map is already fully set.
+			switch v.(type) {
+			case map[string]interface{}:
+				keys := value.Get(fname + ".keys")
+				values := value.Get(fname + ".values")
+				if keys.Exists() && values.Exists() {
+					// an arbitrary map, field is already set.
+					return true
+				}
+			default:
 				// not a map, field is already set.
 				return true
 			}