client handler: allow to set/get application specific data

[drakkan]

This is a common pattern to allow applications to store arbitrary extra data on each connection and easily retrieve this data.

Currently in sftpgo I have something like this map[uint32]any, where the make key is the connection id, accessing this map requires a mutex, adding the extra data directly on the connection allows safe access without a mutex

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0b1ad95) 86.46% compared to head (f3d4994) 86.49%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
+ Coverage   86.46%   86.49%   +0.03%     
==========================================
  Files          11       11              
  Lines        1603     1607       +4     
==========================================
+ Hits         1386     1390       +4     
  Misses        151      151              
  Partials       66       66              

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