refactor(forms): relax [formField] input type from FieldTree to Field

[leonsenft]

FieldTree was an unnecessarily specific type for the [formField] input. It forced the directive to care about what kind of FieldTree was bound–specifically whether it was Reactive Forms compatible or not. This made it difficult to author forms system-agnostic components with passthrough [formField] inputs.

This change introduces Field, which represents the callable state of a FieldTree<TModel> (e.g. () => FieldState<TValue>), without any of its the structure or compatibility concerns.

Use of FieldTree for inputs is still appropriate when the component cares about the field's structure. For example:

@Component({
  template: `
    <street-address [formField]="address.street" />
    <zip-code [formField]="address.zip" />
  `,
})
class Address {
  readonly address = input.required<FieldTree<Address>>();
}

[mmalerba]

reviewed-for: public-api

[leonsenft]

Passing TGP.

[leonsenft]

Note for caretaker: requires patch cl/867757980

[mattrbeck]

This PR was merged into the repository. The changes were merged into the following branches:

• main: 3606902

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