Add a sendMqttClientProxyMessage helper for client proxy implementation

[ianmcorvidae]

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[codecov]

Codecov Report

Attention: Patch coverage is 14.28571% with 6 lines in your changes missing coverage. Please review.

Project coverage is 59.42%. Comparing base (e675050) to head (a097161).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
meshtastic/mesh_interface.py	14.28%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #790      +/-   ##
==========================================
- Coverage   59.50%   59.42%   -0.08%     
==========================================
  Files          24       24              
  Lines        4121     4128       +7     
==========================================
+ Hits         2452     2453       +1     
- Misses       1669     1675       +6     

Flag	Coverage Δ
unittests	59.42% <14.28%> (-0.08%)	⬇️

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