Use child object from sessionStorage JSON as defaultValue in Editor

[MattiasSp]

Description
When using an attribute with a defaultValue set in the editor, e g

[
  {
    "name": "skapad_av",
    "title": "Skapad av: ",
    "type": "hidden",
    "defaultValue": {
      "type": "sessionStorage",
      "key": "loggedInUser",
      "updateOnEdit": false
    }
  }
]

the value in the session storage variable is used as-is. Often, e g when using the OIDC-plugin, the value of interest is actually a child object within a JSON block.

It would be great if there was a possibility to specify what specific object within a JSON parent object is supposed to be used as the default value.

Describe the solution you'd like
A new defaultValue config parameter: keyType.

By default, the current behaviour is kept - getting the variable that matches key exactly. If set to "json_path", a child object can be referenced with simple dot notation, e g "key": "oidc_user.displayname" in the case of the OIDC-plugin.

[
  {
    "name": "skapad_av",
    "title": "Skapad av: ",
    "type": "hidden",
    "defaultValue": {
      "type": "sessionStorage",
      "key": "oidc_user.displayname",
      "keyType": "json_path",
      "updateOnEdit": false
    }
  }
]

Describe alternatives you've considered
Handling some sort of general or JSON-specific attribute transformation before an attribute is applied could be useful sometimes but would be a more complex solution.

EDIT: Updated the proposed solution to reflect the discussion on the last developer meeting.

[tonnyandersson]

Not entirely unrelated to #1237.

[MattiasSp]

Not entirely unrelated to #1237.

I agree, although trying to extend the replacer to also find and return an arbitrary subvalue from JSON would go beyond its intended (and expected) scope in my opinion.

But inspired by your comparison I put the JSON search function into its own utilities file instead :)