Add hide background option to iframe card

[timmo001]

Breaking change

Proposed change

For pages which allow transparent backgrounds, useful to mix with view backgrounds to allow an iframe to blend into the page

Screenshots

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

type: iframe
url: https://example.com/page?background=transparent
hide_background: true

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue or discussion:
• Link to documentation pull request: Document hide background option for iframe card home-assistant.io#41609

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