Backend: Set and use fields to support private container registries

MR: Support adding image-pull-secrets to workspaces (!166343 - merged)

Description

As per the plan in Provide support to use images from private cont... (&14664 - closed) , set and use fields to support private container registries

NOTE - This issue is currently refined based on the assumption that there will be versioning in the agent config table and that any updates would only affect new workspaces - Backend Design Discussion: versioning Agent Con... (#474184 - closed) . If there are any chances in that decision, this issue would have to be relooked at for refinement(maybe de-normalize these values to the workspaces table).

Acceptance Criteria

While setting/updating fields in the agent config

• If image_pull_secret(default []) is provided in the agent config, store it in their corresponding DB column. Else, store their defaults.

While reconciling the workspace, in the response payload

• When responding to the agent
  • send the associated agent's image_pull_secret for each workspace as well in https://gitlab.com/gitlab-org/gitlab/blob/3fca7db9f4de99e64f1d5f3a00b1531718eea5b8/ee/lib/remote_development/workspaces/reconcile/output/response_payload_builder.rb#L46

- [ ] Update integration specs as well if required moved to Backend: Update the deployment to use the image... (#477784 - closed)