Tracking Issue for async http

[arlosi]

Summary

Tracking issue for refactoring work within Cargo migrate networking to use the http_async module.

Unresolved Issues

• - Add http_async module refactor(network): use async for registry network operations #16745
• - Registry index uses http_async refactor(network): use async for registry network operations #16745
• - Crate downloading uses http_async refactor(network): convert .crate downloads to use http_async #16902
• - GitHub "fast path" uses http_async - Convert GitHub fast path to use http_async #16912
• - crates.io crate (registry operations) uses http_async - Remove curl dependency from crates-io crate #16936
• - git2-rs transport uses http_async - Not sure if we want to convert this or not. It's the last use of Easy within Cargo outside http_async.

Future Extensions

Unstable feature for alternate backend such as reqwest.

About tracking issues

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A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.