[#682] Make the paho.Client on mqtt.Adaptor public

[andrewebdev]

This will allow the access to the underlying paho.Client so that we can use it's methods and properties directly, without needing to create new wrappers and methods to access said methods.

[codecov[bot]]

Codecov Report

Merging #683 (94630fd) into dev (da10293) will decrease coverage by 0.00%. The diff coverage is 62.50%.

@@            Coverage Diff             @@
##              dev     #683      +/-   ##
==========================================
- Coverage   77.29%   77.29%   -0.01%     
==========================================
  Files         158      158              
  Lines       10927    10944      +17     
==========================================
+ Hits         8446     8459      +13     
- Misses       2026     2030       +4     
  Partials      455      455              

Impacted Files	Coverage Δ
platforms/mqtt/mqtt_adaptor.go	84.84% <62.50%> (ø)
drivers/spi/apa102.go	78.43% <0.00%> (-5.29%)	:arrow_down:
drivers/gpio/relay_driver.go	92.15% <0.00%> (+1.68%)	:arrow_up:

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[gen2thomas]

I will going to close this PR because:

• making the client exported seems to be not accepted
• there is no progress on this PR since 3 years (e.g. with providing a implementation for getter function)
• the suggested implementation in the related issue #445 is not used