Dynamic Modules Added Cluster Lb Context Set Dynamic Metadata

Added the envoy_dynamic_module_callback_cluster_lb_context_set_dynamic_metadata_number and envoy_dynamic_module_callback_cluster_lb_context_set_dynamic_metadata_string ABI callbacks so custom cluster load balancers can annotate the current request with dynamic metadata at request time inside envoy_dynamic_module_on_cluster_lb_choose_host, mirroring the existing HTTP filter setters. The metadata is readable later on the same request (for example via %DYNAMIC_METADATA(namespace:key)%). The Rust SDK exposes these as ClusterLbContext::set_dynamic_metadata_number and ClusterLbContext::set_dynamic_metadata_string.