feat: add rustls Ring provider feature

[jcarver989]

Summary

• add a public rustls-ring feature parallel to rustls and rustls-no-provider
• pass Ring directly to each rustls ClientConfig without installing a process-global CryptoProvider
• prefer Ring when both provider features are unified
• cover the feature in the CI matrix and test that building a client leaves the process default unset

This allows applications that standardize on Ring to select the provider through Cargo features, while preserving rustls as the AWS-LC-backed default and rustls-no-provider for application-managed providers.

Validation

• cargo fmt --check
• cargo check --no-default-features --features rustls-no-provider
• cargo test --lib --no-default-features --features rustls-ring (47 passed)

[gakonst]

Downstream validation: Nanocodex integrated this change at d45d0c3 to make ring its sole Rustls provider while preserving an embedding host process-level provider. gakonst/nanocodex#86 is merged; full Linux/macOS CI passed, and the six-target nightly published successfully from master SHA 4fd42ec168dce5b1dbacc1612055bc8755dc6505. We also ran cargo test --lib --no-default-features --features rustls-ring here (47 passed). The feature eliminated fresh-process CryptoProvider panics from direct reqwest Client construction and kept aws-lc out of the complete downstream graph.

[gakonst]

^ @seanmonstar any interest in getting this merged / released?

[Absolucy]

oh nice i was just about to do this myself, am tired of manually having to install the ring crypto provider bc i don't want aws-lc-sys nuking my build times lol

[In-line]

Well, at work we were trying to compile 32 bit windows Rust crate and it was failing to build with aws-lc-sys.. Well, trying to disable it in reqwest, let us to do downgrade to reqwest 0.12.X.

I was also on my way to open PR, happy it's already done.