Rails: Add validation for image_pull_secrets when using shared namespace mode

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What does this MR do and why?

• Add model validation in RemoteDevelopment::WorkspacesAgentConfig to ensure every item in image_pull_secrets in the workspace_agent_configs table, the namespace attribute should be the same as shared_namespace column on the workspace_agent_configs table if shared_namespace is not nil.
• Update the unit tests at ee/spec/models/remote_development/workspaces_agent_config_spec.rb and the integration tests at ee/spec/models/remote_development/workspace_spec.rb.

References

Rails: Add validation for image_pull_secrets wh... (#519276 - closed) • Vishal Tak, jp-govau • 18.0

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