Better text customization - Ability to customize Create Account button Text

### 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 would like to be able to customize the text for sign up button using I18n.putVocabulariesForLanguage(). Thank you.

Currently getCreateAccountText() is being used for the sign up button in the SignUp.tsx
`
{getCreateAccountText()}
`

It seems that SIGN_UP_BUTTON is defined in [defaultTexts.ts](https://github.com/aws-amplify/amplify-ui/blob/68c0a0e1c89ede36b4b0bdd7aad8f82f8c5b23b7/packages/ui/src/i18n/dictionaries/authenticator/defaultTexts.ts#L56) which may intentionally create for customization purpose, but it is not being used.

References:
1. [textUtils.tx](https://github.com/aws-amplify/amplify-ui/blob/68c0a0e1c89ede36b4b0bdd7aad8f82f8c5b23b7/packages/ui/src/helpers/authenticator/textUtil.ts#L120)
2. [SignUp.tsx](https://github.com/aws-amplify/amplify-ui/blob/68c0a0e1c89ede36b4b0bdd7aad8f82f8c5b23b7/packages/react/src/components/Authenticator/SignUp/SignUp.tsx#L53)

### 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 packages/react/src/components/Authenticator/SignUp/SignUp.tsx and packages/ui/src/helpers/authenticator/textUtil.ts, then inspect SIGN_UP_BUTTON in packages/ui/src/i18n/dictionaries/authenticator/defaultTexts.ts. Confirm how I18n.putVocabulariesForLanguage() supplies the vocabulary, and verify that the Create Account button displays the customized text.

Written by the indexing model from the issue text.