feat: add foundational X11 forwarding support

[vicajilau]

Summary

• add X11 request support in session/channel APIs
• handle incoming x11 channel-open messages on the client
• add protocol roundtrip tests for x11 and x11-req messages

Context

• Closes X11 forwarding support #1

Validation

• dart test

[codecov]

Codecov Report

❌ Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.96%. Comparing base (5b92bb7) to head (7e7f14a).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/src/ssh_client.dart	0.00%	37 Missing ⚠️
lib/src/ssh_channel.dart	0.00%	8 Missing ⚠️
lib/src/ssh_forward.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   40.77%   40.96%   +0.19%     
==========================================
  Files          60       60              
  Lines        4594     4640      +46     
==========================================
+ Hits         1873     1901      +28     
- Misses       2721     2739      +18     

Flag	Coverage Δ
unittests	40.96% <0.00%> (+0.19%)	⬆️

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