[FR] Ability to tell if form is dirty when adding custom sign up fields

### On which framework/platform would you like to see this feature implemented?

React

### Which UI component is this feature-request for?

Authenticator

### Please describe your feature-request in detail.

We are implementing an "Agree to terms" checkbox on sign up following the example listed in your docs (https://ui.docs.amplify.aws/react/connected-components/authenticator/customization#sign-up-fields).

We noticed that the error message is showing up before the form is submitted, and we would like to have a way to only show the error message is the form is "dirty".

### Please describe a solution you'd like.

_No response_

### We love contributors! Is this something you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request.
- [ ] ⚠️ This feature might incur a breaking change.

Start with the React Authenticator custom sign-up-fields example in the linked documentation and reproduce the Agree to terms checkbox behavior. Trace when its validation error appears relative to form submission, and define completion as showing that error only once the form is dirty. Verify the behavior in the Authenticator sign-up flow.

Written by the indexing model from the issue text.