feat: add course overview field to RTEN model

[andrey-canon]

Description

Add course overview field

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":"$"
            },
            "noShow":"#",
            "expired":"#",
            "ndaRefused":"#",
            "deliveryEventType":"?"
         }
      ]
   },
   "authorization":{
        "clientAuthorizationID": "1-3"
   },
   "vueAppointmentID":"$",
   "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]

LGTM