Airgap Security Artifact Registry - Use CA file for validation

Attempting to use the Configure offline endpoints and air-gapped environments (https://www.elastic.co/guide/en/security/current/offline-endpoint.html).

The self-hosted https server is using a self-signed cert / internal CA. When it attempts to download the new endpoint artifacts, I get:

`[elastic_agent.endpoint_security][error] Http.cpp:327 CURL error 60: SSL peer certificate or SSH remote key was not OK [SSL certificate problem: self signed certificate in certificate chain]`

Feature request would be the option to point to a CA file or text block for trusted ca cert for validation of self-signed certs, alongside the base_url .