api: Implement a public Go API

[daniel-weisse]

Proposed changes

• Implement a public Go API
  • Provides an easy native Go interface for performing CLI commands that interact with the MarbleRun Coordinator
• Use Go API as a backend for CLI commands
  • This removes the /cli/internal/rest package, as http calls are made by the API
    • Cert loading and writing from the rest package was moved to the new /cli/internal/certcache package
• Heavily simplify the code in /internal/attestation and move it to /api/attestation
• Add /api/v2/recover endpoint with better machine readable responses

Additional info

• AB#4009
• I would like to eventually also move the integration test to use the Go API, but as this PR is already quite a bit larger than I originally intended, this will be postponed to a different PR

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