Environment variable substitution in node config

[autodidaddict]

Proposed change

Support the classic environment variable substitution syntax (e.g. ${MY_SECRET}) in the node configuration file. Go through all of the configuration fields where it makes sense and perform the environment variable substitution at load time.

Use case

At some point operating and maintaining Nex might get difficult if the configuration files remain entirely static. In some cases it would be easier if the configuration file referred to an env var so that the same config file could be used in multiple locations but have different variables (e.g. when supplying NATS connections and credentials)

Contribution

No response