Added new table and API for Workflow Events

What does this MR do and why?

This MR introduces the object required to support Human in the loop for Duo Workflows. We need to be able to store human events and associate them to the workflow.

Refer to this issue for a detailed design: #487382 (comment 2102734264)

• Added new table and API for Workflow Events
• Added new table for Workflow Events and an API to be able to insert events

MR acceptance checklist

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Enable duo_workflows feature flag
2. Create a workflow (this can be done via the IDE)
3. Test creating an event with the following API call

curl -X POST \
    -H "Authorization: Bearer $GITLAB_TOKEN"  \
    -H "Content-Type: application/json" \
    -d '{
        "event_type":"message"
        "message": "test message"
    }' \
    http://gdk.test:3000/api/v4/ai/duo_workflows/workflows/<workflow_id>/events

4. Test fetching existing events

curl -X GET \
    -H "Authorization: Bearer $GITLAB_TOKEN"  \
    http://localhost:3000/api/v4/ai/duo_workflows/workflows/<workflow_id>/events

5. Test updating the status of an event

curl -X PUT \
    -H "Authorization: Bearer $GITLAB_TOKEN"  \
    -H "Content-Type: application/json" \
    -d '{
        "event_status": "Delivered"
    }' \
    http://localhost:3000/api/v4/ai/duo_workflows/workflows/<workflow_id>/events/1