Arti 2.1.0 advances relay and RPC support, overhauls configuration

Arti 2.1.0 introduces significant progress on relay and RPC development, with preparations underway for future middle relay testing. The release also features a comprehensive overhaul of the configuration system using a derive-deftly-based approach. Arti’s minimum supported Rust version has been raised to 1.89.0, and several bugfixes and infrastructure improvements are included.