feat: Add Private Runner for CI-CD builds

Closes https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/23828

This change adds a private runner for performing CI/CD builds for the team.

How does it work?

1. Deploys the GitLab Runner helm chart to the ai-assist-test cluster
2. Adds a new private runner to this project: see https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/runners/23970581
3. Adds the build tag, ai-assist-container-build for the new private runner.
4. Adds a new node pool (min 0, max 1) of e2-standard-8 machines. These machines have 8 cores, 32GB of RAM.
5. Switches docker image builds from Docker to Kaniko using the infrastructure template: https://gitlab.com/gitlab-com/gl-infra/common-ci-tasks/-/blob/main/kaniko.md
6. Uses the build tag (ai-assist-container-build) to build images on the new node pool.