add session closing callback API

[wyfo]

See #2008

[github-actions]

PR missing one of the required labels: {'dependencies', 'documentation', 'breaking-change', 'bug', 'new feature', 'internal', 'enhancement'}

[github-actions]

PR missing one of the required labels: {'new feature', 'documentation', 'breaking-change', 'internal', 'bug', 'dependencies', 'enhancement'}

[codecov]

Codecov Report

Attention: Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.99%. Comparing base (6f9cfa0) to head (663cbb5).

Files with missing lines	Patch %	Lines
zenoh/src/api/session.rs	97.22%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2009      +/-   ##
==========================================
- Coverage   71.15%   70.99%   -0.16%     
==========================================
  Files         367      367              
  Lines       61866    61901      +35     
==========================================
- Hits        44019    43949      -70     
- Misses      17847    17952     +105     

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