Introduce N8N functionality to support custom certificates

[arvchristos]

Documentation at https://docs.n8n.io/hosting/configuration/configuration-examples/custom-certificate-authority indicates that when the directory /opt/custom-certificates exists, those certs should be added to the trusted CAs.

This PR introduces the same logic as in https://github.com/n8n-io/n8n/blob/master/docker/images/n8n/docker-entrypoint.sh , which tests for the existence of this directory and attempts to install the supported CA certs.