Fine-grained permissions with impersonation in CI/CD tunnel

What this post added

This post details the implementation of fine-grained permissions for the CI/CD Tunnel using impersonation. It explains how to configure the `access_as` attribute in the Agent's configuration file to impersonate the CI job itself or specific user/system accounts within the Kubernetes cluster. The post provides a step-by-step guide, including setting up a GKE cluster, configuring sample projects, connecting the GitLab Agent, and demonstrating the deployment of an Ingress application using default impersonation and then with `access_as:ci_job`.