feat: add dial_args() for socket-ready addresses (fixes #114)

[acul71]

Summary

• Add dial_args(ma) -> (network, address) for thin-waist IP and unix multiaddrs.
• Export, tests, README, and printable example.
• Fixes No dial_args() function for converting multiaddrs to socket-ready strings #114.

Demo

=== dial_args() ===
/ip4/1.2.3.4/tcp/80
  -> network='tcp4', address='1.2.3.4:80'
...

Test plan

• pytest tests
• newsfragment 114.feature.rst
• python examples/dial_args/dial_args_example.py

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