Fix part of errors in building demos.

[wanghaEMQ]

• Fix the error in building the mqttv5 demo.
• Fix the error in building the mqtt demo.
• Add a new interface to get reason codes from unsuback.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -0.22 ⚠️

Comparison is base (1f74bff) 61.57% compared to head (c32bd66) 61.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #149      +/-   ##
==========================================
- Coverage   61.57%   61.35%   -0.22%     
==========================================
  Files          99       99              
  Lines       26439    26453      +14     
==========================================
- Hits        16279    16231      -48     
- Misses      10160    10222      +62     

Impacted Files	Coverage Δ
src/supplemental/mqtt/mqtt_msg.c	27.52% <0.00%> (-0.60%)	⬇️
src/supplemental/mqtt/mqtt_public.c	16.40% <0.00%> (-0.21%)	⬇️

... and 17 files with indirect coverage changes

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