Add C ABI wrapper and reusable Rust TLS client

[pomm10]

Why

Expose wreq to C# and provide a reusable Rust TLS client facade modeled after the existing Go TlsClient implementation.

What changed

• Add a production-oriented C ABI with compatible Request/Response layouts, exported request execution and response/error ownership APIs, and a C header.
• Add C# P/Invoke declarations and a minimal native usage example.
• Add TlsClient, TlsClientPool, header/header-order parsing, ID-based client reuse, proxy and timeout configuration, redirects, complete response collection, and explicit handling for unsupported named TLS profiles.
• Document the Rust TLS client behavior and current profile limitations.

Validation

• cargo fmt --all -- --check
• cargo metadata --no-deps
• git diff --check
• Focused cargo tests were attempted but could not complete because the environment lacks libclang.dll required by btls-sys.