Make packet capture dependencies optional

[keilogic]

Summary

• move native packet-capture/interface dependencies into a capture extra
• keep packet capture imports lazy with actionable install hints when the extra is missing
• update install docs and package metadata so base installs avoid the broken pcapy metadata path

Fixes #14.

Validation

• python -m py_compile setup.py chains\sources\packet_streamer.py chains\utils\net_utils.py
• python -m flake8 setup.py chains\sources\packet_streamer.py chains\utils\net_utils.py --max-line-length=140
• python setup.py --name
• git diff --check
• fresh venv: pip install . succeeded with only dpkt
• fresh venv import smoke: net_utils.inet_to_str(...) worked, missing capture helpers raised the new install hints
• fresh venv dry-run: pip install '.[capture]' --dry-run resolved pcapy-ng and netifaces