Change dialog content from List to concrete object

[oskogstad]

Description

Instead of Content being a list:

"content": [
            {
              "type": "TITLE",
              "mediaType": null,
              "value": [
                {
                  "value": "Fin tittel",
                  "languageCode": "nb"
                }
              ]
            },
            ...

Content as an object with named properties for each content type:

"content": {
   "title": {
       "value": [ { "value": "Fin tittel", "languageCode": "nb" } ]},
       "mediaType": "text/plain"
   "summary": { ...},
   "extendedStatus": { ... },
}

This will make the API much easier to understand and use for front-end

Implementation

Make a new model class for the APIs and map from the abstract lookup type
Maybe add test to ensure these two match, so that if the lookup type is updated, the outer facing class is updated as well

Tasks

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1. Implementation tasks are added here
2. Prepare documentation (if relevant - either update working document, or add a new file in docs)
3. Add e2e-test (if relevant)

Threat modelling

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1. Does this change introduce any potential security issues?

Acceptance criteria

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[elsand]

As an added bonus, this will make PATCH operations much cleaner since one does not have to know the actual index of the content you want to change. Do note though, that we want to have this content on transmissions as well, where there will be different validation rules, which might affect nullability of some of these fields.