Gitlab Agent installation: Provide custom image repository

Problem to solve

Could it be possible to configure the repository from where gitlab agent image is pulled?

At the moment the installation method via one-line or kpt doesn't have the ability to set the image repository. It's hardcoded to registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/

I would like it to pull from a private registry repository (harbor)

Proposal

Make the image repository customizable via a KPT flag, and expose this through the CLI as --agentk-image-repository.

Example CLI help text after the proposed changes:

Prints the YAML manifests based on specified configuration

Usage:
  cli generate [flags]

Flags:
      --agent-image-repository string Repository from where the agentk image is pulled (default "registry.gitlab.com/gitlab-org/cluster-integration/gitlab-agent/agentk")
      --agent-token string            Access token registered for agent
      --agent-version string          Version (tag) of the agentk image to use (default "v14.7.0")
  -h, --help                          help for generate
      --kas-address string            GitLab Kubernetes Agent Server address
      --name-prefix string            The prefix to use for names of Kubernetes objects
      --namespace string              Kubernetes namespace to create resources in (default "gitlab-agent")
      --no-rbac                       Do not include corresponding Roles and RoleBindings for the agent service account