Debugger should support authentication with symbol server and source link

In 1.15, the debugger added support for Source Link and Symbol Server. However, it currently doesn't support either symbol servers or source link endpoints that require authentication.

This feature request tracks adding support for either or both of these. My guess is that we would want to do both at the same time, but I could be wrong.

### Workaround
Manually clone the repo/source repository and add a [`sourceFileMap`](https://github.com/OmniSharp/omnisharp-vscode/blob/master/debugger-launchjson.md#source-file-map) entry to your launch.json.