Add support for passing objects reference to FlexBox

[philippjfr]

Fixes #6384

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 71.62%. Comparing base (f01fe1f) to head (27e1d46).
Report is 1 commits behind head on main.

Files	Patch %	Lines
panel/layout/flex.py	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6387      +/-   ##
==========================================
+ Coverage   71.60%   71.62%   +0.01%     
==========================================
  Files         305      305              
  Lines       45594    45605      +11     
==========================================
+ Hits        32649    32663      +14     
+ Misses      12945    12942       -3     

Flag	Coverage Δ
unitexamples-tests	71.62% <85.71%> (+0.01%)	⬆️

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