TLS Configuration using env variables

[arieltorti]

Right now the only way to configure TLS certificates is to use WithTLS with a tlsConfig object. I am aware #1160 removed the ability to setup SSL using env variables in favor of using listeners or modifying fnserver. This means that any change made to the SSL configurations will require a re-compilation, which is impractical to say the least.

I propose to add at least two basic env variables FN_CERT_FILE and FN_KEY_FILE to allow for a more flexible configuration. I will be willing to implement it if its accepted.

As a side note: what do you think about creating an endpoint for dynamically modifying configuration variables without needing to restart the node/server ?