Support HTTP headers for connecting to Coder behind an authenticating proxy

[spikecurtis]

From our meeting notes:

Passing HTTP headers into the manager/tunnel

• literal headers can specified by end users feat: support user-supplied literal headers #24
• header commands need to be either
  - [preferred] cleared by admins, e.g. written to Admin only config (Registry on Windows)
  - run as the end user account that specified it