Implement most specific `group/project` selection logic

Logic is described in https://gitlab.com/gitlab-org/cluster-integration/gitlab-agent/-/blob/master/doc/kubernetes_ci_access.md#apiv4joballowed_agents-api

This page may contain information related to upcoming products, features and functionality. It is important to note that the information presented is for informational purposes only, so please do not rely on the information for purchasing or planning purposes. Just like with all projects, the items mentioned on the page are subject to change or delay, and the development, release, and timing of any products, features, or functionality remain at the sole discretion of GitLab Inc.