[improve] Validate user paths in Functions utils

[merlimat]

Motivation

For Python & Go functions, validate that the filename provided does not look into a parent directory.

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository:

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.74074% with 16 lines in your changes missing coverage. Please review.

Project coverage is 73.24%. Comparing base (bbc6224) to head (6164a8a).
Report is 334 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #22833      +/-   ##
============================================
- Coverage     73.57%   73.24%   -0.33%     
- Complexity    32624    32652      +28     
============================================
  Files          1877     1889      +12     
  Lines        139502   141780    +2278     
  Branches      15299    15559     +260     
============================================
+ Hits         102638   103848    +1210     
- Misses        28908    29926    +1018     
- Partials       7956     8006      +50     

Flag	Coverage Δ
inttests	27.44% <0.00%> (+2.86%)	⬆️
systests	24.85% <0.00%> (+0.53%)	⬆️
unittests	72.26% <40.74%> (-0.59%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
...org/apache/pulsar/broker/web/ExceptionHandler.java	100.00% <100.00%> (ø)
...he/pulsar/functions/utils/FunctionConfigUtils.java	72.27% <0.00%> (-0.69%)	⬇️
.../storage/filesystem/FileSystemPackagesStorage.java	58.62% <55.55%> (-9.38%)	⬇️

... and 381 files with indirect coverage changes

[lhotari]

#23985 depends on this change, so cherry-picking branch-3.0 and branch-3.3