Add `scan_execution_policies` endpoint to the Kubernetes internal API

Why are we doing this work

To facilitate allowing scan execution policies to enforce scans on agents, we need to provide the Agent with an API endpoint which it can use in order to retrieve security policies.

Relevant links

Non-functional requirements

• Documentation: Document this endpoint in the internal API documentation
• Feature flag:
• Performance:
• Testing: See !69022 (merged) for an example of how this can be tested

Implementation plan

Add a GET internal/kubernetes/modules/starboard_vulnerability/scan_execution_policies endpoint to the internal API at ee/lib/ee/api/internal/kubernetes.rb. This endpoint will:

1. Return a 404 error if the security_orchestration_policies feature is not available
2. Use PolicyFinder (to be added with #347066 (closed)) in order to query for the policies affecting agent.project and return them in the API response.