Add icon to Entities card schema

[ZephireNZ]

Proposed change

Add the icon to the Entities card's schema. Currently if a card includes an icon, then the card cannot be edited at all using the UI editor as the schema is considered invalid.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New feature (thank you!)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Example configuration

Additional information

Split off from #8804

Checklist

• The code change is tested and works locally.
• There is no commented out code in this PR.
• Tests have been added to verify that the new code works.

If user exposed functionality or configuration variables are added/changed:

• Documentation added/updated for www.home-assistant.io