Add scan_execution_policies endpoint to the Kubernetes internal API

What does this MR do and why?

Addresses #356729 (closed)

This MR adds a new internal API endpoint (GET /api/v4/internal/kubernetes/modules/starboard_vulnerability/scan_execution_policies) for Kubernetes agent to facilitate allowing scan execution policies to enforce scans on agents.

The API returns the scan_execution_policies configured through the security policy project for the project the agent is configured to. The API skips the authorization check for :read_security_orchestration_policies as the internal API won't have current_user.

How to set up and validate locally

1. Create a new KAS JWT using the rails console:

   JWT.encode({ 'iss' => Gitlab::Kas::JWT_ISSUER }, Gitlab::Kas.secret, 'HS256')

2. Create a new gitlab project and configure security policy project following docs and create a new policy using the editor

3. Create new agent:

   agent = Clusters::Agent.new(project_id: project.id, created_by_user: User.find(1), name: "test-agent")
   agent.save!

4. Create new agent token and retrieve the value

   token = Clusters::AgentToken.new(agent: agent, created_by_user: User.find(1), name: "test-agent-token")
   token.save!
   token.token

5. Send the API request:

    curl --header "Gitlab-Kas-Api-Request: <JWT from step 1>" \
    --header "Authorization: Bearer <token from step 4>" \
    --header "Content-Type: application/json" \
    --url "http://gdk.test:3000/api/v4/internal/kubernetes/modules/starboard_vulnerability/scan_execution_policies" \ 

   Reponse:

   {
     "policies": [
       {
         "name": "Policy",
         "description": "Policy description",
         "enabled": true,
         "yaml": "---\nname: Policy\ndescription: 'Policy description'\nenabled: true\nactions:\n- scan: container_scanning\nrules:\n- type: pipeline\n  branches:\n  - main\n",
         "updated_at": "2022-06-02T05:36:26+00:00"
       }
     ]
   }

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