Support adding image-pull-secrets to workspaces

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

Backend: Set and use fields to support private ... (#474989 - closed)

Backend: Add fields to support private containe... (#477777 - closed)

What does this MR do and why?

This MR:

• adds migrations to introduce image_pull_secrets to the workspace config along with appropriate validations.
• configures the settings module to handle image_pull_secrets
• saves image_pull_secrets from agent updates
• forwards workspaces agent config associated image_pull_secrets in the case of a full reconciliation and empty array in the case of a partial reconciliation

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 - closed)

MR acceptance checklist

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

Screenshots or screen recordings

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Before	After

How to set up and validate locally

• Start GDK with workspaces setup
• Create a workspace and access it. Observation: You can access the workspace
• On the rails console, search for the agent config associated with the workspace you created (RemoteDevelopment::WorkspacesAgentConfig.all gives all configs). Observation: Accessing the field image_pull_secrets will provide you with an empty array
• Update your workspace's agent config by adding the following image_pull_secret lines like so:

  network_policy:
    .......
  image_pull_secrets:
    - name: example-image-pull-secrets-a
      namespace: default
    - name: example-image-pull-secrets-b
      namespace: default

• Observation update is successful (check rails console to see the update on image_pull_secret)
• Trigger a full reconciliation ( you could do this by reducing the time here in settings) and access your workspace. Observation: You can access the workspace

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