`RemoteStashCustomData`: stashing via custom script, made possible

[khsrali]

Part of https://github.com/aiidateam/aiida-core/issues/6764,

Has to be re-based after merging PR #6772 More later..

[codecov[bot]]

Codecov Report

:x: Patch coverage is 89.87342% with 8 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 79.20%. Comparing base (1e8cd0b) to head (de02775). :warning: Report is 53 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aiida/engine/daemon/execmanager.py	66.67%	5 Missing :warning:
src/aiida/orm/nodes/data/remote/stash/custom.py	90.33%	3 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6777      +/-   ##
==========================================
+ Coverage   79.13%   79.20%   +0.07%     
==========================================
  Files         565      566       +1     
  Lines       43394    43461      +67     
==========================================
+ Hits        34334    34417      +83     
+ Misses       9060     9044      -16     

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[khsrali]

@agoscinski Please review :bowing_man:

[khsrali]

@agoscinski documentation is in place at https://github.com/aiidateam/aiida-core/pull/6936 Please approve here, if no more comments. thanks