Initial API model with Create Correspondence focus

[RagnarFatland-Avanade]

Initial API model with Create Correspondence focus for discussion.

Description

Added controllers for Attachments and Correspondence. Attachments can be Initialized / uploaded separately and thus shared among different Correspondences / Correspondence Services. A Correspondence can have 0-multiple attachments. Notifications can be created as part of a correspondence, but only according to pre-defined templates.

Discussion points:

• Should Message body also become an Attachment?
  • Implemented as that for now.
• Should attachments be named "content" or "contentparts"
  • Additional field to specify their functional role, such as "HumanReadableMessageBody", "humanReadAbleAttachment", etc?
  • Yes
• Land a stadardized method to point to the data location, or use similar structure to Dialogporten?https://digdir.github.io/dialogporten/#dialog-create-request
  • Url for now
• Expand with "Correspondence Templates" and "Mass Sending" functionality, building on the Shared Attachment concept?
  • in future version,
• Misc "TODO's" in code need to be resolved.
  • Tweaking in future versions of API.

Related Issue(s)

• 42

Verification

• [x] Your code builds clean without any errors or warnings

Documentation

• [ ] User documentation is updated with a separate linked PR in altinn-studio-docs. (if applicable)