Tracking: Simplify download and/or TLS backends

Part of https://github.com/rust-lang/rustup/issues/1611.

Motivation

The direct motivation of this change is to simplify Rustup's building process in order to make Rustup more cross-build friendly and prevent surprise stable-only CI build errors like the ones we've encountered while bumping OpenSSL to v3, i.e. https://github.com/rust-lang/rustup/pull/3478, https://github.com/rust-lang/rustup/pull/3668, https://github.com/rust-lang/rustup/issues/4707, etc.

Status quo

• The default currently is to use reqwest with native-tls (OpenSSL on Linux, native stacks on macOS and Windows)
• If RUSTUP_USE_CURL is set, we use the curl backend, which defaults to openssl
• If RUSTUP_USE_RUSTLS is set, we use reqwest with rustls

https://github.com/rust-lang/rustup/pull/3788#issuecomment-2076819462

Goal

The eventual goal would be removing RUSTUP_USE_CURL and/or RUSTUP_USE_RUSTLS, leaving a single stack "in favor of maximally-Rust solutions", while "making sure we minimize the risk of getting users stuck".

To minimize frictions during this change, some necessary refactoring patches will be developed in the meantime, with a primary focus on improving Rustup's observability by interfacing with the tracing ecosystem.

https://github.com/rust-lang/rustup/pull/3788#issuecomment-2076679922

Tasks

• https://github.com/rust-lang/rustup/pull/3793
• https://github.com/rust-lang/rustup/pull/3868
• https://github.com/rust-lang/rustup/pull/3803
• https://github.com/rust-lang/rustup/pull/3879
• https://github.com/rust-lang/rustup/pull/3873
• https://github.com/rust-lang/rustup/pull/3871
• https://github.com/rust-lang/rustup/pull/3877
• https://github.com/rust-lang/rustup/pull/3891
• https://github.com/rust-lang/rustup/pull/3798
• https://github.com/rust-lang/rustup/pull/3903
• https://github.com/rust-lang/rustup/pull/3815
• https://github.com/rust-lang/rustup/pull/3788
• #4277

Review parent issue #1611 and the task pull requests listed here, especially the backend and tracing refactors; this issue names no file or test entry point. Done means the download/TLS backend plan advances toward a single maximally-Rust stack while minimizing the risk of users getting stuck and preserving the cross-build goals.