feat(io): support mqtt5

[ngjaying]

Refactor to extract mqtt connection interface
Implement the interface for v4 and v5 separately
Fix subscription lock

Closes #1940

[codecov]

Codecov Report

Attention: Patch coverage is 79.31034% with 84 lines in your changes missing coverage. Please review.

Project coverage is 71.11%. Comparing base (410f52d) to head (c7b396c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/io/mqtt/v5client/client.go	59.43%	58 Missing and 13 partials ⚠️
internal/io/mqtt/v4client/client.go	91.46%	5 Missing and 2 partials ⚠️
internal/io/mqtt/conn.go	95.56%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3384      +/-   ##
==========================================
+ Coverage   71.08%   71.11%   +0.03%     
==========================================
  Files         403      405       +2     
  Lines       46080    46321     +241     
==========================================
+ Hits        32754    32938     +184     
- Misses      10753    10800      +47     
- Partials     2573     2583      +10     

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