Update and finalize Coder VPN RFC

[sreya]

We've spent the last couple of sprints doing technical validation work for MacOS and Windows so that we don't encounter any huge surprises when we tackle the feature in earnest. Now that we have a good idea of what sort of technical hurdles await us, we should update the RFC and move towards getting it approved by all stakeholders.

Some things I think would be good to add:

• Update the design for the MacOS implementation
• Update the design for the Windows implementation
• Provide some phases that demonstrate how the feature will be tackled from initial implementation to GA. Ideally broken down by what is expected to be delivered sprint-to-sprint.