feat(deps): add rustls support to reqwest for musl compatibility

Replace native-tls (OpenSSL) with rustls-tls in the workspace reqwest
dependency to enable cross-compilation to musl targets without requiring
OpenSSL cross-compilation toolchain.

This change adds `rustls-tls` and `rustls-tls-webpki-roots` features to
the reqwest workspace dependency, which propagates to all crates that
use reqwest via the workspace, including google-cloud-gax-internal.

Benefits:
- Eliminates OpenSSL dependency and cross-compilation complexity
- Enables musl target builds without vendored OpenSSL or cargo-zigbuild
- Reduces binary size (rustls is pure Rust, no C dependencies)
- Improves musl compatibility and static linking support

The change maintains API compatibility as reqwest's rustls backend
provides equivalent functionality to native-tls for HTTP client operations.

Author: casibbald

Cargo.toml

@@ -314,7 +314,7 @@ prost                 = { default-features = false, version = "0.14", features =
 prost-build           = { default-features = false, version = "0.14" }
 prost-types           = { default-features = false, version = "0.14" }
 rand                  = { default-features = false, version = "0.9.2" }
-reqwest               = { default-features = false, version = "0.12.24", features = ["json"] }
+reqwest               = { default-features = false, version = "0.12.24", features = ["json", "rustls-tls", "rustls-tls-webpki-roots"] }
 rustls                = { default-features = false, version = "0.23" }
 rustls-pemfile        = { default-features = false, version = "2" }
 serde                 = { default-features = false, version = "1.0.228", features = ["serde_derive"] }