Support setting additional root certificates on supported platforms.

To use, add a new entry `additional_trust_anchors` to `package.json` containing a list of PEM-encoded certificates (i.e. `"-----BEGIN CERTIFICATE-----\n...certificate data...\n-----END CERTIFICATE-----\n"`).
These certificates are used as additional root certificates for validation, to allow connecting to services
using a self-signed certificate or certificates issued by custom CAs.