Provide types for the Integration Fields API

[angeloashmore]

Is your feature request related to a problem? Please describe.

Developers writing custom Integration Field integrations do not have any Prismic-provided types to use. First-party provided types would make the development process easier.

Describe the solution you'd like

The Integration FIelds API is used to send data to Prismic to populate Integration Fields.

Details on the API are provided here: https://prismic.io/docs/core-concepts/integration-fields-setup

Similar to the types provided in api.ts, we could provide types for the Integration Fields API. This would be helpful when writing custom Integration Field integrations.

Describe alternatives you've considered

The types can be manually typed in a project. Not ideal.

Additional context

This request was prompted after prismicio/prismic-types#38 was discovered.