Add creation timestamp to pre-detections

[ppcad]

The timestamp of pre-detections is the same as the timestamp of the event it was created from.
This change adds an additional creation_timestamp to pre-detections that shows when the pre-detection was created by Logprep.

[dtrai2]

I just checked the changes. I only have two suggestions, one regarding the for-loop and could you also please add a CHANGELOG.md entry? Thank you!

[dtrai2]

I am not really available for the next two weeks. @ekneg54 could you set the final approve after my changes are implemented?

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.09% 🎉

Comparison is base (11f1442) 92.28% compared to head (f7a7497) 92.38%.
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #436      +/-   ##
==========================================
+ Coverage   92.28%   92.38%   +0.09%     
==========================================
  Files         133      135       +2     
  Lines        9595     9541      -54     
==========================================
- Hits         8855     8814      -41     
+ Misses        740      727      -13     

Files Changed	Coverage Δ
logprep/processor/pre_detector/processor.py	96.15% <100.00%> (+2.15%)	⬆️

... and 14 files with indirect coverage changes

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