Add support for constant in ha-form

[bramkragten]

Proposed change

Backend PR: home-assistant/core#37241

Closes: #6284

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

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:

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

[balloob]

When a form is loaded with constant values, we should make sure we add those to the values that we're going to send to the server.

[balloob]

Backend merged