feat: add candidate field to RTEN model

[andrey-canon]

Description

This adds a new User field to the RTEN model so that the record is associated with a User record in the database. This includes the required view modifications

Testing instructions

1. Run migrations ./manage.py lms migrate eox_nelp
2. set the setting ENABLE_CERTIFICATE_PUBLISHER, this will enable the result notification pipeline and the external certificate publisher, its not necessary to test the external call so you can use an invalid mode, or if you want to test the external call set CERTIFICATE_PUBLISHER_VALID_MODES with the desired mode
3. Make a postman call with the following data

{
   "eventType":"RESULT_AVAILABLE",
   "eventTime":"?",
   "candidate":{
      "candidateName":{
         "firstName":"Maximo",
         "lastName":"Tercero"
      },
      "email":"?",
      "lastUpdate":"?",
      "vueCandidateID":"?",
      "clientCandidateID":"?"
   },
   "attempt":"$",
   "exams":{
      "exam":[
         {
            "examDefinition":{
               "examSeriesCode":"?",
               "examLanguageCode":"?",
               "examName":"?",
               "deliveryModel":"?",
               "vueExamVersionId":"?",
               "vueExamRevisionID":"?",
               "clientExamVersionId":"?",
               "examForm":"?",
               "isBetaVersion":"#"
            },
            "examResult":{
               "outcomeType":"?",
               "startTime":"?",
               "timeUsed":"$",
               "passingScore":"0.8",
               "score":"0.832",
               "rawScore":"$",
               "grade":"0.5",
               "correct":"$",
               "incorrect":"$",
               "skipped":"$",
               "unscored":"$"
            },
            "noShow":"#",
            "expired":"#",
            "ndaRefused":"#",
            "deliveryEventType":"?"
         }
      ]
   },
   "vueAppointmentID":"$",
   "clientCode":"?",
   "vueCandidateID":"$",
   "clientCandidateID":"?" /// Complete this
}

4. Check in admin panel

Before

After

Additional information

Include anything else that will help reviewers and consumers understand the change.

• Does this change depend on other changes elsewhere?
• Any special concerns or limitations? For example: deprecations, migrations, security, or accessibility.
• Link to other information about the change, such as Jira issues, GitHub issues, or Discourse discussions.

Checklist for Merge

• Tested in a remote environment
• Updated documentation
• Rebased master/main
• Squashed commits

[johanseto]

Do you think we could use blank=True, in order to don't be mandatory in forms or serializer.?

[andrey-canon]

I wanted to do this as mandatory as possible since the idea is that every record has a user however we don't know if the client will send the whole data in the example format and secondly the migration process complains about previous records when the null=True field is not present, that's why I added the null field and I don't think we should include the blank field

[johanseto]

Ideally, we'd avoid modifying clientCandidateId(product requirement?) due to its impact on product functionality and user experience. Any changes would necessitate significant code updates related to anonymous IDs.

[johanseto]

Tested and working in my local env.