perf python: Port net_dropmonitor to perf module

Port net_dropmonitor.py from tools/perf/scripts/python/ to a standalone
script in tools/perf/python/:
- Refactor the script into a DropMonitor class with full type
  annotations to encapsulate state.
- Use perf.session for skb:kfree_skb event processing and add argparse
  CLI support (-i/--input and -k/--kallsyms).
- Resolve kernel drop addresses via perf.session symbols/callchains and
  binary search over /proc/kallsyms, ignoring zeroed kptr_restrict
  addresses with graceful fallback when kallsyms is unavailable.
- Remove Python 2 compatibility code.

Add a shell test (test_net_dropmonitor_python.sh) to verify the
standalone script.

Assisted-by: Antigravity:gemini-3.1-pro
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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