Create ADR for proposed Welsh forms API implementation

[DavidBiddle]

PR Checklist

• If you are proposing a new decision record document, used the right template for that
  - (ADR, decision-record, engagement, research)
• Set yourself as the Assignee
• Tag anyone you would like to review, or @forms-design or @forms-devs
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What

Proposes an approach for how forms-runner gets translated forms from forms-api.

Outcome of spike: https://trello.com/c/Ok9i8oup/2401-spike-how-will-forms-runner-get-translated-forms-from-forms-api

When to review

Ideally I'd like to have a card for implementing this ready by the end of sprint 24, so reviews by Wednesday 13 August would be ideal if possible 💐

How to review

1. Semantic: Do you agree with the changes? Does the recommended approach make sense to you?
2. Syntactic: Spelling, grammar, etc.

Who can review

Anyone

[DavidBiddle]

Spike branches for option 1:

• forms-api: https://github.com/alphagov/forms-api/tree/spike-add-welsh-columns-to-api-snapshot
• forms-runner: https://github.com/alphagov/forms-runner/tree/read-translated-fields-from-json

Spike branches for option 2:

• forms-api: https://github.com/alphagov/forms-api/tree/spike-translate-snapshot-at-request-time
• forms-runner: https://github.com/alphagov/forms-runner/tree/request-language-from-api

Both spikes use a rebased version of the original Mobility spike branch as a base.

There's no spike branch for option 3 at the moment - I thought of this after I'd investigated and started writing up the other two, and it's a lot more complex to look into. If people think it's a better approach than the one proposed in this ADR, we can make a spike branch to investigate it further next week. The runner implementation would be the same aas option 2, but I think we'd have to make quite a few changes in admin and API.