core-modular: Support `userSettingGroupKey` for custom storage keys in user settings

[JohannesDoberer]

Description

The userSettingGroupKey property on a user settings group config allows storing user settings data under a custom key, independent of the group's object key in the configuration. This is currently not supported in core-modular / the UI5 library.

Use case

Multiple user settings groups (e.g. themingIframe and themingWC) need to read/write to the same storage namespace (e.g. theming). Without userSettingGroupKey, each group can only store data under its own object key.

Expected behavior

When a group has userSettingGroupKey: 'custom3', the storeUserSettings callback should receive data keyed under custom3, not under the config object key.

userSettingGroups: {
  myPanel: {
    label: 'My Panel',
    userSettingGroupKey: 'custom3', // store under 'custom3'
    viewUrl: '...',
  }
}
// storeUserSettings receives: { custom3: { ... } }

Current behavior

The userSettingGroupKey property is ignored. Data is always stored under the object key (myPanel in this example).

Original test reference

luigirepo/test/e2e-test-application/cypress/e2e/tests/1-angular/user_settings_dialog.cy.js — test: "Test userSettingGroupKey"