Support cancelation in getSession

<!-- ⚠️⚠️ Do Not Delete This! bug_report_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Also please test using the latest insiders build to make sure your issue has not already been fixed: https://code.visualstudio.com/insiders/ --> <!-- Use Help > Report Issue to prefill these. --> - VSCode Version: 1.51.1 - OS Version: Steps to Reproduce: 1. Using the example code github-authentication-sample. Open extension and execute command 'Github authentication example', then a prompt in browser shows 2.close the page 3. In vs code, execute the command again 4. Prompt didn't showup(1.50.1 doesn't have the issue) <!-- Launch with `code --disable-extensions` to check. --> Does this issue occur when all extensions are disabled?: Yes