Support custom system-wide TLS options

[fredmaggiowski]

The feauture or bug you are proposing

Since #84 serviceBuilder support custom tls options; it would be a good feature to be able to configure the library for system-wide TLS configurations

The description of the bug or the rationale of your proposal

The use case we'd like to address is providing the user with the ability to configure their services with custom TLS certificates for all serviceProxies that are created; we figured that providing environment variables with file paths (that should be mounted as files, e.g k8s configmaps/secrets) would be a good solution; if these variables are provided the library will read them at boot and provide all serviceBuilder invocation with the proper tls options.

Note: If a serviceBuilder provides specific tls options they should override the system wide

[davidebianchi]

System wide certificates could be added without changes in lib, using node env variable NODE_EXTRA_CA_CERTS?

[fredmaggiowski]

NODE_EXTRA_CA_CERTS works for Certification Authories certificates is there an equivalent for client private key and certificate? 🤔