UI: Update schema default attribute

[etschelp]

A schemas default attribute determines how data related to the schema is labeled in the UI. Currently the UI only supports setting the default attribute when a schema is created, the default attribute is then marked with an *

Backend Endpoints:

PUT /api/admin/schema/{id} POST /api/admin/schema

{
  "defaultAttributeName": "something"
}

The following features are missing:

1. Option to update a schemas default attribute

Add the option to the following dialog:

2. Option to set the default attribute when importing an json-ld schema

Probably another checkbox on the left hand side same as the attribute selector, but one that only allows a single selection.

3. Option to set the default attribute when importing an Indy schema

This is a bit more tricky as there is a preview schema functionality needed to make this work (same as with json-ld), which involves backend work and changing the flow in the component.

[DritonGoxhufi]

Option 1 is resolved in #773

Step 1: Open an existing schema and click on the edit-button

Step 2: choose new default attribute via checkbox

Step 3: save/update the state into the store