HTTPX Fiber Concurrency Plugin: Seamlessly Share Connections Across Fibers

The HTTPX :fiber_concurrency plugin allows a single session's connections to be used across fibers managed by a fiber scheduler, ideal for long-lived connections. It's required by default when using the :persistent plugin. Example code shows launching multiple fibers within a thread, each concurrently using `http.get()` to access a URL. This plugin is essential for programs using fiber schedulers, such as those built with the async gem.