Add support for secure storage of HTTP proxy credentials to pass into language server

Problem to solve

The simple fix for Support proxy authentication (#1234 - closed) was supporting the passthrough of credentials inside of the http.proxy URL.

VS Code's built-in proxy prompt doesn't actually expose a way to read credentials from the API though so we should consider introducing a UX/UI for prompting the user upon a 407 error status response. We would securely store the credentials like we do for OAuth/PAT.

Proposal

Prompt for username/password to pass through to the language server process environment.

Further details

Links / references