Add agent project authorizations to finder

What does this MR do and why?

Implements the last remaining bit to surface project level authorizations for the GitLab Kubernetes Agent CI/CD tunnel.

See #327850 (closed)

How to set up and validate locally

1. GDK needs a runner attached, and the gitlab-k8s-agent enabled
2. Enable the feature flag group_authorized_agents:

   Feature.enable(:group_authorized_agents)

3. Add an agent with configuration project gitlab-agent-testing/config with the following configuration:

   # config/.gitlab/agents/first/config.yaml
   ci_access:
     projects:
       - id: gitlab-agent-testing/ci-project # <-- We will run CI/CD in here
         access_as:
           agent: {}

4. In a separate project, gitlab-agent-testing/ci-project, add the following .gitlab-ci.yml:

   # .gitlab-ci.yml
   test:
     image: registry.gitlab.com/gitlab-org/cluster-integration/helm-install-image/releases/3.6.3-kube-1.17.17-alpine-3.14
     script:
       - kubectl config get-contexts

5. View the job logs for the test job

Screenshots or screen recordings

The job logs should look like this (the failed jobs you see on the side were hurdles with getting my local runner working):

MR acceptance checklist

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