Dynamic Modules Added A Formatter Extension

Added a dynamic modules formatter extension (envoy.formatter.dynamic_modules) that allows implementing custom %COMMAND% operators for access logs and header formatting in a dynamic module. The formatter context exposes request, response, and trailer headers, stream info attributes, dynamic metadata, the local reply body, and the access log type. The Rust SDK exposes this through the formatter module and the formatter: arm of declare_all_init_functions!. See :ref:DynamicModuleFormatter <envoy_v3_api_msg_extensions.formatter.dynamic_modules.v3.DynamicModuleFormatter> for configuration details.