Tracking: Simplify download and/or TLS backends

[rami3l]

Part of #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. #3478, #3668, 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

#3788 (comment)

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.

#3788 (comment)

Tasks

Beta Give feedback

1. chore(deps): unpin openssl version #3793
   dependencies +1
2. refactor(test): execute all #[rustup_macros::unit_test]s within a tokio context #3868
   +0
3. chore(dist/features): ship tracing and friends by default #3803
   +0
4. Reserve RUSTUP_LOG for tracing usages instead of RUST_LOG #3875
   enhancement +1
5. docs(dev-guide/tracing): mention RUSTUP_LOG and console-based tracing #3879
   +0
6. feat(log): unhide tracing::instrument from behind feature = "otel" #3873
   +0
7. Pass Process around explicitly #3871
   +0
8. Process cleanup #3877
   +0
9. Docs: Update dev-guide according to recent Process API changes #3970
   +0
10. test(clitools): revive run_inprocess() #3891
    +0
11. feat(download-backend)!: make reqwest/rustls the new default #3798
    +0
12. feat(download/rustls): use rustls-platform-verifier #3903
    +0
13. CfT: Test out Rustup's reqwest backend with rustls #3806
    2 of 2
    help wanted meta +2
14. feat(download): reflect the download/TLS backends in the user agent #3815
    +0
15. refactor(download): remove curl backend #3788
    +0
16. Replace custom download crate with reqwest #993
    help wanted +1

[djc]

Also, many benefits from rustls just being more secure by default than OpenSSL.