docs(eventhubs): add C++ FFI interop guide

[j7nw4r]

Summary

Adds sdk/eventhubs/azure_messaging_eventhubs/docs/cpp_interop.md, a reference guide for consuming the Rust Event Hubs SDK from a modern C++ (C++20) application through a Rust extern "C" FFI shim.

Motivation

azure_messaging_eventhubs is a pure-Rust crate with an async API and no C ABI, so it cannot be linked from C++ directly. There was no documentation describing the FFI path. The guide fills that gap and serves as a template for building such a shim.

Changes

• New doc with eight sections: why a shim is required, architecture, the Rust FFI shim crate, cbindgen header generation, the modern C++ RAII wrapper, a complete sample program, build/link (CMake) instructions, and a safety/threading/lifetime contract.
• Shim covers the producer (open/send_event/close), consumer, and receiver paths against the real crate API, including a process-wide Tokio runtime, opaque handles, an errno-style thread-local error channel, and catch_unwind panic guards.
• Documents the self-referential receiver problem (stream_events(&self) borrows the receiver) and a sound resolution via boxing plus struct field drop order, with ouroboros/self_cell noted as safe alternatives.
• C++ layer uses unique_ptr RAII handles, status-codes-to-exceptions, std::string_view in and std::span<const std::byte> out.

Documentation only; no code or build changes.