Add data manager endpoint to check if a user exists and is eligible in the activate account system
Description
Add a new endpoint to the activate account data manager API that can be called to check whether a user exists and is eligible for activation in the API.
This endpoint only needs to respond with a 200 if the user does exist, or a 404 if they do not. (And other 403, 500 etc. responses as normal).
Further details
Task list
Acceptance criteria
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Endpoint exists to check if a user is eligible in the activate account system