[BUG] [GOLANG] XML support not fully implemented

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Generated golang structs do not have required XML tags to marshal/unmarshal correctly. For the given OpenAPI spec below the follow structs are generated:

...               
// Document struct for Document
type Document struct {                                                    
        Name string `json:"name"`                                         
        Item DocumentItem `json:"item"`             
}            
...
// DocumentItem struct for DocumentItem                                   
type DocumentItem struct {                                                
        Type string `json:"type"`                                         
        Value string `json:"value"`                                       
} 
...

Which have no XML tags at all. So the features detailed here https://swagger.io/docs/specification/data-models/representing-xml/ don't work.

The expected generated code looks something like:

...
// Document struct for Document
type Document struct {
        XMLName xml.Name `xml:"document"`                                                    
        Name string `json:"name" xml:"name"`                                         
        Item DocumentItem `json:"item" xml:"item"`             
}            
...
// DocumentItem struct for DocumentItem                                   
type DocumentItem struct {                                                
        Type string `json:"type" xml:"type,attr"`                                         
        Value string `json:"value" xml:"value"`                                       
} 
...

openapi-generator version

5.3.0

OpenAPI declaration file content or URL

openapi: 3.0.3
info:
  version: 1.0.0
  title: test-api

paths:
  /foo:
    get:
      responses:
        200:
          description: OK
          content:
            application/xml:
              schema:
                $ref: "#/components/schemas/document"

components:
  schemas:
    document:
      type: object
      required:
      - name
      - item
      properties:
        name:
          type: string
        item:
          type: object
          required:
          - type
          - value
          properties:
            type:
              type: string
              xml:
                attribute: true
            value:
              type: string

Generation Details

Steps to reproduce

openapi-generator generate -i testapi.yaml -g go

Related issues/PRs

Suggest a fix

Add appropriate xml tags when generating structs.

Start by running openapi-generator generate -i testapi.yaml -g go with the OpenAPI declaration in the issue and inspect the generated Document and DocumentItem structs. Trace the Go generator's model and XML handling from that output, then verify that generated structs include the expected XMLName, element, and attribute tags and marshal/unmarshal correctly.