Add packet drop filter (#419)

bpf/flows.c

bpf/flows_filter.h

bpf/headers/vmlinux_amd64.h

bpf/headers/vmlinux_arm64.h

bpf/headers/vmlinux_ppc64le.h

bpf/headers/vmlinux_s390.h

bpf/network_events_monitoring.h

bpf/pca.h

bpf/pkt_drops.h

bpf/rtt_tracker.h

bpf/types.h

bpf/utils.h

docs/flow_filtering.md

pkg/agent/agent.go

pkg/agent/config.go

pkg/agent/packets_agent.go

pkg/decode/decode_protobuf.go

pkg/ebpf/bpf_arm64_bpfel.go

pkg/ebpf/bpf_arm64_bpfel.o

pkg/ebpf/bpf_powerpc_bpfel.go