Agent: Add config to support for private container registries

MR: Update proto definitions to include image_pull_... (gitlab-org/cluster-integration/gitlab-agent!1757 - merged)

Description

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

remote_development:
  image_pull_secrets:
    - name: example-image-pull-secrets
      namespace: default

Acceptance Criteria

• Agent's protobuf definition has a new field image_pull_secrets which is a list of name(string) and namespace(string). The default value of image_pull_secrets would be []. No other validations are required.