Add support for Security Policies enforcing Cluster Image Scanning scan

Why?

This MR adds new functionality to read scan execution policies from GitLab and enforce Cluster Image Scanning with the given policy configuration. Security policies should take precedence over the agent config.yml for security scans. The policies are retrieved from Rails via an API endpoint implemented in gitlab-org/gitlab!94218 (merged). Currently we will support only one policy created for given agent.

Screenshots / Video

Demo: https://youtu.be/tm4_SwBrwWk

How to test it?

1. Create local k3d cluster
2. In your local GDK create new project, configure your gitlab agent (remember your agent-name as it will be needed in the policy) in this repository.
3. Go to Security & Compliance -> Policies, click on New policy and select Scan execution policy
4. Create new policy and merge new MR:

   type: scan_execution_policy
   name: "Enforce CIS"
   description: "This policy is enforcing cluster image scanning test"
   enabled: true
   rules:
     - type: schedule
       cadence: "0 * * * *"
       agents:
         agent-name:
           namespaces:
             - default
   actions:
     - scan: cluster_image_scanning

5. Run kas and agent, the scan should be executed.

NOTE: You have to have gitlab-agent service account in your local cluster to create vulnerabilities in your local GitLab instance.