[BUG] Custom Attributes are not applied to escalated/merged Alert Assets and IOCs

[StaSys846]

Describe the bug
When an IRIS Alert with asset/IOC information is merged to an existing case or escalated to a new case, Asset and IOCs included for import to the case don't include custom_attribute definitions (e.g. additional GUI elements like tabs/custom fields in the Asset/IOC modal view).

To Reproduce
Steps to reproduce the behavior (prerequisite: custom_attributes are defined for IOCs/Assets):

1. Go to Alerts view
2. Click on Merge (into new or existing case)
3. Select alert assets and/or IOCs to be imported in the the selected case
4. Assets and IOCs are imported to the case, BUT custom attributes are not applied

Expected behavior
After alert merging/escalation to a case including assets and/or IOCs, the asset/IOC data should include the defined custom_attributes

Desktop (please complete the following information):

• OS: ubuntu 22.04
• Browser: Not browser dependent
• Version 2.4.7

Additional context
In a manual modal view "add asset" the custom attributes are loaded by "get_default_custom_attributes" (e.g. #163 case_asset_routes.py) and provided as additional input to the modal view render template, so that custom attributes are rendered for the "add" modal view and added to the asset data. This default_custom_attribute handling is missing for assets/iocs included in alerts.