Added support for optional Authorization and Accept headers via environment variables.

[azchohfi]

Fixes #633.

This allows me to call:

$env:WINGET_CREATE_HTTP_CLIENT_AUTH_TOKEN = "{MY_PAT}"
$env:WINGET_CREATE_HTTP_CLIENT_ACCEPT_HEADER = "application/octet-stream"
winget new

Then provide an url such as: https://api.github.com/repos/myorg/privaterepo/releases/assets/ID

Microsoft Reviewers: Open in CodeFlow

[mdanish-kh]

I'm not sure I understand the use case here. The manifests submitted to microsoft/winget-pkgs require that the URL is publicly available. If the InstallerUrl is under a private repo which needs an auth header, the manifest would not pass the validation pipelines at winget-pkgs, which means you will have to make the repo public anyway. Is the intent to submit the wingetcreate generated manifest to a private GitHub repository other than microsoft/winget-pkgs?