Add async single-flight proxy resolution

[chrmarti]

Summary

• add a Tokio-gated resolve_proxy_async API backed by one lazy dedicated resolver thread
• coalesce identical URL/config-generation lookups so concurrent callers await one result without occupying blocking threads
• queue distinct targets through a bounded channel, with panic and cancellation-safe in-flight cleanup
• retain the existing synchronous API for runtime-agnostic and callback consumers

Validation

• cargo test
• cargo test --features tokio
• cargo clippy --all-targets --features tokio -- -D warnings
• cargo fmt --all -- --check
• concurrency test: 32 simultaneous same-URL calls on a current-thread Tokio runtime start exactly one blocking resolution

[Copilot]

Pull request overview

This PR adds a Tokio-gated asynchronous proxy-resolution API that avoids blocking async runtime workers by funneling resolution through a lazily created dedicated background thread, while coalescing identical concurrent lookups (“single-flight”) and bounding queued work.

Changes:

• Add ProxyResolver::resolve_proxy_async (Tokio feature) implemented via a per-resolver background worker plus in-flight coalescing keyed by URL + config generation.
• Expose a crate-level resolve_proxy_async convenience wrapper for the global resolver.
• Update docs and add a Tokio concurrency test validating identical-call coalescing.

Show a summary per file

File	Description
src/types.rs	Makes Error clonable to support sharing async results through channels.
src/resolver.rs	Implements the async worker thread, in-flight coalescing, bounded queueing, and adds a Tokio test.
src/lib.rs	Exposes a new Tokio-gated resolve_proxy_async wrapper on the global resolver.
README.md	Documents the new async API and recommended usage under the tokio feature.

Review details

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• Files reviewed: 4/4 changed files
• Comments generated: 4
• Review effort level: Low