Add ha-form context

[balloob]

Breaking change

Proposed change

Add a new feature to selectors to allow specifying a context.

Each selector can define their own supported context keys. The values of context in a configuration are set to field names. <ha-form> will process context and substitute field names with the value of the field.

Currently only implemented in the attribute picker.

Important requirement: if context is used, the selector is responsible to adjust the value if a new context value is passed in. So in the case of attribute picker, if a new entity is passed in, set value to undefined if the new entity does not support the picked attribute.

const schema = [
      { name: "entity", selector: { entity: {} } },
      {
        name: "Attribute",
        selector: { attribute: { entity_id: "" } },
        context: { filter_entity: "entity" },
      },
]

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 or discussion:
• 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