Implement ADR-14 "JWT library free jwt user generation"

[matthiashanel]

Overview

In select clients (java, csharp, go) with the highest impact, implement the utility function IssueUserJWT.

The behavior is documented in ADR-14.

Clients and Tools

• [ ] Go @piotrpio
• [x] Java @scottf
• [x] .Net @scottf
• [x] Javascript - https://github.com/nats-io/jwt.js

Other Tasks

• [ ] docs.nats.io updated (@matthiashanel )
• [ ] Update ADR to Implemented (@matthiashanel )
• [ ] Update client features spreadsheet

Client authors please update with your progress. If you open issues in your own repositories as a result of this request, please link them to this one by pasting the issue URL in a comment or main issue description.