feat: Migrate job, waiter deprecation, togglable init container

[khvn26]

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Changes

This PR does the following:

• Removes the wait-for-db init container as Flagsmith's entrypoint integrates this functionality.
• Adds an ability to remove the migrate-db init container.
• Adds an ability to deploy a migrate-db Kubernetes job, and customise it with extra containers and volumes.

This, along with #184, helps the chart users to add containers responsible for database connectivity, such as Google's cloud-sql-proxy.

How did you test this code?

This code now works as part of one of our enterprise on-premise deployments.