Add V3 endpoint for fetching delivery configurations

[DavidBiddle]

What problem does this pull request solve?

Trello card: https://trello.com/c/P7bDQZTP/3248-get-submission-delivery-configuration-from-latest-live-version-of-the-form-in-forms-runner

Adds a new endpoint to the V3 API to get the draft or live/archived delivery configurations for a form.

It's at /api/v3/forms/#{form.id}/delivery-configurations/{state}, where #{state} is either draft or current. Calling it with draft will return the delivery config from the draft form document, while current will return the config for the most recent live or archived form document.

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[github-actions]

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