Draft: add image pull secrets to db with appropriate defaults and validation

Issue: DB: Add fields to support private container reg... (#474988)

What does this MR do and why?

This MR adds migrations to introduce image_pull_secrets to the workspace config along with appropriate validations. We are introducing this field as part of our epic to support private container registry access for workspaces Provide support to use images from private cont... (gitlab-org#14664)

MR acceptance checklist

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Before	After

How to set up and validate locally

• Start GDK with workspaces setup
• Create a workspace agent config
• On the rails console, search for the agent config you created (RemoteDevelopment::WorkspacesAgentConfig.all gives all configs)
• Observation accessing the field image_pull_secrets will give you an empty array
• Update a field in the config on git like dns_zone
• Observation update is successful (check rails console to see the update)

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