Update vets-api to support request-level callbacks

[sarahwcodes]

Business Need

We received a DVSA request to setup a callback for a new service (check_in in vets-api, but it's actually the VEText service). While the notification-api only allows one callback endpoint per service, we do have the option of using a request-level callbacks as provided to us by the Llamas. We will need to update our vets-api module to support these request-level callbacks going forward. We should be able to do so without having to involve any existing services, since this will be a behind-the-scenes update (and once it's in place, it means we no longer will need to provide support via the configuration step).

User Story(ies)

As a developer of the portal
I want to have both request- and service-level callbacks available to me
So that I can use service-level callbacks more selectively and skip the configuration step altogether

Additional Info and Resources

• Request-level callback docs
• UK gem (#send_email and #send_sms for reference): https://github.com/alphagov/notifications-ruby-client/blob/93756391ca90b555285182167dbb72eb07608652/lib/notifications/client.rb#L35-L48
• Keep in mind that we will need to support old and new methods of callbacks at the same time

Engineering Checklist

Required

• Create a new feature flag: va-notify-request-callbacks
• Update internal #send_email and #send_sms to accept another arg (method definitions in the UK gem look like they don't do any filtering out of args, but we'll need to monkey patch if they do)
• Update VANotify::CallbacksController
• Update vets-api documentation
• Add callback URL to settings (. . .)

Acceptance Criteria

Required

• Changes are behind a feature flag and feature flag is toggled on
• When feature flag is turned on, we do not see errors coming from request-level callbacks
• Docs are updated
• Changes for enabling request-level callbacks are merged in

QA Considerations

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n/a, BE change

Potential Dependencies

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Out of Scope

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