[Go][Experimental] Add discriminator support to anyOf (#6511)

* add discriminator support in anyof

* update samples

modules/openapi-generator/src/main/resources/go-experimental/model_anyof.mustache

@@ -15,6 +15,35 @@ func (dst *{{classname}}) UnmarshalJSON(data []byte) error {
 	}
 
 	{{/isNullable}}
+	{{#discriminator}}
+	{{#mappedModels}}
+    {{#-first}}
+	// use discriminator value to speed up the lookup
+	var jsonDict map[string]interface{}
+	err := json.Unmarshal(data, &jsonDict)
+	if err != nil {
+		return fmt.Errorf("Failed to unmarshal JSON into map for the discrimintor lookup.")
+	}
+
+    {{/-first}}
+	// check if the discriminator value is '{{{mappingName}}}'
+	if jsonDict["{{{propertyBaseName}}}"] == "{{{mappingName}}}" {
+		// try to unmarshal JSON data into {{{modelName}}}
+		err = json.Unmarshal(data, &dst.{{{modelName}}});
+		if err == nil {
+			json{{{modelName}}}, _ := json.Marshal(dst.{{{modelName}}})
+			if string(json{{{modelName}}}) == "{}" { // empty struct
+				dst.{{{modelName}}} = nil
+			} else {
+				return nil // data stored in dst.{{{modelName}}}, return on the first match
+			}
+		} else {
+			dst.{{{modelName}}} = nil
+		}
+	}
+
+	{{/mappedModels}}
+	{{/discriminator}}
 	{{#anyOf}}
 	// try to unmarshal JSON data into {{{.}}}
 	err = json.Unmarshal(data, &dst.{{{.}}});
@@ -44,4 +73,4 @@ func (src *{{classname}}) MarshalJSON() ([]byte, error) {
 	return nil, nil // no data in anyOf schemas
 }
 
-{{>nullable_model}}
+{{>nullable_model}}

samples/openapi3/client/petstore/go-experimental/go-petstore/model_gm_fruit.go

@@ -101,3 +101,4 @@ func (v *NullableGmFruit) UnmarshalJSON(src []byte) error {
 	return json.Unmarshal(src, &v.value)
 }
 
+