Background Migration to populate GroupPushRule with historical PushRule records

What does this MR do and why?

This MR adds a batched background migration that will populate GroupPushRule table with historical data from the PushRule table.

The initial work to fix the trigger so PushRule data can be copied as it has been merged here: !199043 (merged)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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

Query Performance

• Total historical GroupPushRule currently in PushRule to be migrated: 230,896 (https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41906/commands/128527)
• Select 100 PushRule records per sub-batch using a simulated query to return 100 records from the select query https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41906/commands/128529
• Insert simulated 100 fetched PushRule records into GroupPushRule: https://console.postgres.ai/gitlab/gitlab-production-main/sessions/41906/commands/128530

Related to #498935 (closed)