Propagate cancellation of gRPC requests

[njhill]

@dtrifiro added a partial change for this while ago which was reverted to maintain compatibility with older vLLM version.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.94%. Comparing base (20db20b) to head (e7d17dc).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #130      +/-   ##
==========================================
+ Coverage   55.64%   56.94%   +1.30%     
==========================================
  Files          24       25       +1     
  Lines        1488     1533      +45     
  Branches      269      277       +8     
==========================================
+ Hits          828      873      +45     
+ Misses        583      582       -1     
- Partials       77       78       +1     

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