Support proxy definition for fetch operation by using custom http client using proxy env var or flags to command line

Bug description

I am attempting to install a quarto extension for section bibliographies into a Quarto Book project using PowerShell.
https://github.com/pandoc-ext/section-bibliographies#quarto

When I attempt to do so, I receive this error

My project has my company's http_proxy and https_proxy defined in the .Rprofile

It is not clear from quarto help how the user would verify quarto can see these proxy settings.

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.
• Please provide the output of quarto check so we know which version of quarto and its dependencies you're running.

Start with the failing Quarto extension install command in PowerShell and the quarto help proxy-documentation gap; the issue names no files or tests, so locate the fetch operation and HTTP-client entry point first. Done means proxy environment variables or command-line flags work for the fetch and users can verify the active proxy settings.