Stat Sinks Added A Dynamic Modules Stats Sink

Added a new dynamic modules stats sink extension (envoy.stat_sinks.dynamic_modules) that delegates metric flushing and histogram observations to a dynamic module loaded via dlopen. On each flush the module receives a snapshot of the counters, gauges, and text readouts, plus a callback for every completed histogram sample. Metric names are decoded directly into a module-provided buffer to avoid intermediate allocations. The Rust and Go SDKs expose this through a StatSink interface and a matching stat sink init function. See :ref:DynamicModuleStatsSink <envoy_v3_api_msg_extensions.stat_sinks.dynamic_modules.v3.DynamicModuleStatsSink> for configuration details.