[Cilium v1.19] feat: rewrite DNS plugin with native cilium/ebpf

[nddq]

Summary

Part 4 of the Cilium v1.19 upgrade (#1788).

Replaces inspektor-gadget DNS tracer with a native eBPF socket filter using cilium/ebpf. New eBPF C program, gopacket-based DNS parsing, concurrent worker goroutines.

Files (9):

• pkg/plugin/dns/_cprog/dns.c (NEW), pkg/plugin/dns/_cprog/doc.go (NEW)
• pkg/plugin/dns/dns_bpfel_x86.go (NEW), pkg/plugin/dns/dns_linux.go, pkg/plugin/dns/dns_linux_test.go, pkg/plugin/dns/types_linux.go
• pkg/module/metrics/dns.go, pkg/module/metrics/dns_test.go
• test/plugin/dns/main_linux.go

Estimated size: ~700 lines

Depends on: #2034
Parent issue: #1788