feat!: Narrow `FormField.choices` and `.inputs` types to their implemations

[justlevine]

What

This PR updates the schema types for the FormField.choices and FormField.inputs fields to their respective implementing type, instead of the interface.

Why

... on OptionField {
  adminLabel
  canPrepopulate
  choices {
  ... on OptionFieldChoice { # 👈 This is unnecessary and redundant. We **know** this b/c its on the OptionField
    formattedPrice
    isSelected
    price
    text
    value
  }

  # ✔️ Now we can query it directly on the `choice`
  formattedPrice
  isSelected
  price
  text
  value
}

How

Uses the new Utils::overload_graphql_field_type to avoid the validation issues in graphql_register_field()

Testing Instructions

Additional Info

Checklist:

• [x] This PR is tested to the best of my abilities.
• [x] This PR follows the WordPress Coding Standards.
• [x] This PR has proper inline documentation.
• [x] This PR has unit tests to verify the code works as intended.
• [x] The changes in this PR have been noted in CHANGELOG.md