feat: add dynamic SOCKS5 forwarding API

[vicajilau]

Summary

• add SSHClient.forwardDynamic(...) to provide local dynamic forwarding (ssh -D style)
• implement SOCKS5 server support for NO AUTH + CONNECT over existing SSH forwarding channels
• add configurable hardening options via SSHDynamicForwardOptions:
  • handshakeTimeout
  • connectTimeout
  • maxConnections
• add public dynamic forwarding types in forwarding API surface
• add platform split implementation (dart:io implementation + unsupported stub for non-io platforms)
• add runnable example: example/forward_dynamic.dart
• document usage and quick curl verification in README
• add changelog entry for dynamic forwarding

Notes

• Scope intentionally limited to SOCKS5 NO AUTH + CONNECT for MVP.
• Not available on web runtimes without dart:io.

Validation

• dart analyze
• test suite passed (189 passed, 0 failed)

Refs #30

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.41%. Comparing base (c310497) to head (080d6d4).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/dynamic_forward_io.dart	96.71%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   46.10%   48.41%   +2.30%     
==========================================
  Files          61       63       +2     
  Lines        4880     5040     +160     
==========================================
+ Hits         2250     2440     +190     
+ Misses       2630     2600      -30     

Flag	Coverage Δ
unittests	48.41% <96.87%> (+2.30%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/src/dynamic_forward.dart	100.00% <100.00%> (ø)
lib/src/ssh_client.dart	17.66% <100.00%> (+3.54%)	⬆️
lib/src/ssh_forward.dart	72.22% <100.00%> (+72.22%)	⬆️
lib/src/dynamic_forward_io.dart	96.71% <96.71%> (ø)

... and 3 files with indirect coverage changes

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