Http Drain Timeout Jitter

Added :ref:drain_timeout_jitter <envoy_v3_api_field_extensions.filters.network.http_connection_manager.v3.HttpConnectionManager.drain_timeout_jitter> to HttpConnectionManager. When set, the drain grace period (between the shutdown notice GOAWAY and the final GOAWAY) is extended by a random amount up to drain_timeout * jitter / 100, staggering the final GOAWAY across simultaneously-draining connections to mitigate thundering-herd reconnects.