Add Natvis visualizations for some `http` types

[ridwanabdillahi]

This change adds Natvis visualizations for some http types to help improve the debugging experience on Windows.

Debugging types such as Request and/or Response under a Windows debugger, WinDbg or the Visual Studio debugger, it can be a bit difficult to dissect exactly what is data is contained in each type. The Rust compiler does have Natvis support for some types, but this is limited to some of the core libraries and not supported for external crates.

RFC 3191 proposes adding support for embedding debugging visualizations such as Natvis in a Rust crate. This RFC has been approved, merged and implemented.

Natvis is a framework that can be used to specify how types should be viewed under a supported debugger, such as the Windows debugger (WinDbg) and the Visual Studio debugger.

This PR adds:

• Natvis visualizations for some of the core types such as types in the http::header, http::request, http::response, and http::uri modules.
• Tests for testing visualizers embedded in the http crate.
• Updates to the CI pipeline to ensure tests for visualizers are run so they do not break silently.
• A new debugger_visualizer feature for the http crate to enable the unstable debugger_visualizer feature.

[ridwanabdillahi]

Hello @LucioFranco do you have any time to review this PR? This change adds Natvis visualizations for some of the core http types to ease debugging these types on Windows debuggers along with documentation on how to do so.