pprofile: introduce SetStack helper method

[dmathieu]

This introduces the SetStack helper method for pprofile, as we already have SetFunction, SetMapping, SetString, ...

This is needed to help with setting up merging of profiles. #13106

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.62%. Comparing base (5174026) to head (e06726b).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
pdata/pprofile/stacks.go	50.00%	4 Missing and 2 partials ⚠️

❌ Your patch status has failed because the patch coverage (50.00%) is below the target coverage (95.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #14007   +/-   ##
=======================================
  Coverage   91.61%   91.62%           
=======================================
  Files         655      656    +1     
  Lines       42790    42805   +15     
=======================================
+ Hits        39200    39218   +18     
+ Misses       2766     2764    -2     
+ Partials      824      823    -1     

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