feat: add from_net_addr/to_net_addr helpers (fixes #115)

[acul71]

Summary

• Add from_net_addr() / to_net_addr() with explicit transport="tcp"|"udp".
• Export, tests, README, and printable example.
• Fixes No from_net_addr() / to_net_addr() functions for socket address conversion #115.

Demo

=== from_net_addr() ===
('1.2.3.4', 80) transport=tcp -> /ip4/1.2.3.4/tcp/80
...
=== to_net_addr() ===
/ip4/1.2.3.4/tcp/80 -> ('1.2.3.4', 80)

Test plan

• pytest tests
• newsfragment 115.feature.rst
• python examples/net_addr/net_addr_example.py

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