Restore the Serialize/Deserialize impls for ObserverMode

[maoe]

These impls got removed in 99f16b4 perhaps by accident I suppose. We've been using these impls in our project so I'd like to restore them.

[codecov-commenter]

Codecov Report

Base: 93.39% // Head: 93.38% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (85bb181) compared to base (fd8b15f).
Patch coverage: 0.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #334      +/-   ##
==========================================
- Coverage   93.39%   93.38%   -0.01%     
==========================================
  Files         119      119              
  Lines       18975    18976       +1     
==========================================
  Hits        17721    17721              
- Misses       1254     1255       +1     

Impacted Files	Coverage Δ
argmin/src/core/observers/mod.rs	93.12% <0.00%> (-0.36%)	⬇️

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[stefan-k]

I don't know what I was thinking when I removed this. Apologies for the inconvenience and thanks for reporting and fixing! :) I will release 0.8.1 including this fix soon-ish.

[maoe]

Thanks for the quick turnaround!