Track Direct Transfer and Offline Transfer entity-level lifecycle events

What does this MR do and why?

Fires five internal events on BulkImports::Entity for the Direct Transfer / Offline Transfer entity lifecycle:

• start_gitlab_migration_entity
• finish_gitlab_migration_entity
• fail_gitlab_migration_entity
• cancel_gitlab_migration_entity
• timeout_gitlab_migration_entity

All five fire from after_transition hooks in the entity's state machine, alongside the existing *_project_import / *_group_import calls.

How it differs from the project/group events

• Fires for both project and group entities.
• Can fire before a Project or Group record exists (e.g. an entity that fails during its very first pipeline) — identifiers are attached only when present.
• start_gitlab_migration_entity fires at entity.start!, before any pipeline runs — earlier than start_project_import/start_group_import, which wait for the loader to create the destination. That gap is the point: it counts every attempt, not just the ones that got far enough to create something.
• Carries parent_entity_id so top-level entities can be told apart from nested ones when measuring re-attempts.

label (gitlab_migration vs offline_transfer) and hashed property reuse the shared import_event_attributes helper introduced in !254680 (merged).

Scope

• Part of #617884.
• Closes #627876 (closed).
• Follows the pattern established in !253895 (merged), !253927 (merged), and !254680 (merged).

How to validate locally

1. Enable Snowplow Micro:

gdk config set snowplow_micro.enabled true
gdk reconfigure
gdk restart

2. Monitor http://gdk.test:9091/micro/good.
3. Trigger a Direct Transfer or Offline Transfer migration (group or project). Confirm the five entity events fire alongside the existing *_project_import / *_group_import events.

MR acceptance checklist

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Related to #627876 (closed)