Support HTTP headers for connecting to Coder behind an authenticating proxy

From our meeting notes:

**Passing HTTP headers into the manager/tunnel**
- [x] literal headers can specified by end users https://github.com/coder/coder-desktop-macos/pull/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

Start by tracing how the manager/tunnel receives literal HTTP headers, using the completed work in coder-desktop-macos/pull/24 as context. Determine where header commands are handled and how Admin-only configuration is represented on macOS. Done means command-supplied headers are either cleared by administrators or executed as the end user who configured them.

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