fix(api): declare this parameter type in observable callbacks

[legendecas]

Which problem is this PR solving?

Fixes #3293

Declare the this parameter types for the observable callbacks.

This should not be a breaking change as usages without this parameter to be the expected one are not working anyways.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

• Existing code should continue to be able to compile without any touch.

Checklist:

• Followed the style guidelines of this project

[codecov]

Codecov Report

Merging #3497 (f904549) into main (eecb800) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

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##             main    #3497   +/-   ##
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  Coverage   93.75%   93.75%           
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  Files         249      249           
  Lines        7610     7610           
  Branches     1587     1587           
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  Hits         7135     7135           
  Misses        475      475