Opinionated NATS Conn Helper

[jordan-rash]

I have found myself copy and pasting this function in a handful of projects, so I thought I would PR for consideration into orbit.

This will create you a nats.Conn based on the configuration you provide, which is really helpful when you want to support a handful of auth methods in a CLI or something.

The hierarchy is

1. NatsContext - If provided, uses NATS context configuration
2. NatsCredentialsFile - Uses a credentials file
3. NatsUserJWT + NatsUserSeed - JWT-based authentication
4. NatsUserNkey + NatsUserSeed - NKey-based authentication
5. NatsUser + NatsUserPassword - Basic username/password authentication