Attestation Proxy Client

Create attestation proxy client for copilot.

The attestation proxy will be an optional copilot integration, and likely a completely separate application. Copilot will leverage a REST client in order to query an attestation proxy based on a predefined behavior. Copilot will not need to know how each attestation service works, which will simplify the integration.

Some pilot agents will be deployed in environments where users wish to leverage local platform attestation services such as [KeyLime](https://keylime.dev/). Copilot will query the attestation proxy to ask simply if a given `pilot_agent_id` has been attested before issuing a requested KEK. This will be an AND decision combined with the standard authorization procedure.

Failure to attest will result in a 403 forbidden response, just like a `kek_authorized: false` decision from the database.

Comienza localizando el procedimiento de autorización KEK de Copilot y el punto de entrada del cliente REST; el issue no menciona archivos ni pruebas. Aclara el contrato del proxy y el límite de integración, y después verifica que una pilot_agent_id no atestiguada produzca la decisión 403 descrita junto con el resultado de autorización existente.

Escrito por el modelo de indexación a partir del texto del issue.