ci: enhance coverage reporting and add integration coverage script

[qzhuyan]

• add signal handling in r-proxy and s-proxy to ensure cover data are dumped.
• ensure 'ignored' tests are covered.
• ensure io tests against broker.emqx.io

[codecov-commenter]

Codecov Report

❌ Patch coverage is 95.65217% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.87%. Comparing base (2cb2db5) to head (a0a0ce9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
mqtt_grpc_duality/src/bin/r-proxy.rs	93.93%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main      #82       +/-   ##
===========================================
+ Coverage   58.70%   74.87%   +16.17%     
===========================================
  Files          65       64        -1     
  Lines       17235    15027     -2208     
===========================================
+ Hits        10117    11251     +1134     
+ Misses       7118     3776     -3342     

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