Send email to group owners after direct transfer

What does this MR do and why?

Send email to group owners after direct transfer

This MR triggers an email after a direct transfer enters any one of the three final states. The email is sent to all owners on the parent group.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

1. Enable the flag

   Feature.enable(:notify_owners_of_finished_direct_transfer)

2. Enable local direct transfers

   ApplicationSetting.first.update(
     allow_local_requests_from_web_hooks_and_services: true,
     bulk_import_enabled: true
   )

3. Perform a direct transfer from your GDK to itself.
4. Validate that the email described in #458115 (closed) is sent to the owner of the new group in http://gdk.test:3000/rails/letter_opener

Queries

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/28794/commands/89800:

SELECT
    "bulk_import_entities".*
FROM
    "bulk_import_entities"
WHERE
    "bulk_import_entities"."bulk_import_id" = 13
    AND "bulk_import_entities"."source_type" = 0
    AND "bulk_import_entities"."parent_id" IS NULL
ORDER BY
    "bulk_import_entities"."id" ASC
LIMIT 1

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/28794/commands/89802:

SELECT
    "bulk_import_entities".*
FROM
    "bulk_import_entities"
WHERE
    "bulk_import_entities"."parent_id" IS NULL