examples: add flattened enum deserialization

[Frankkkkk]

Add a small example about flattened enum deserialization. This is a small workaround around serde's serde's
issue.

Big thanks to @Mingun for (most) of the code + help!

[Mingun]

Thanks for the example. Need to fix lint issues and find a way to enable "serialize" feature for this example or just run it when that feature is active.

[Frankkkkk]

It should be fixed, sorry for the oversight. Cheers
EDIT: 🤦 sorry

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 64 lines in your changes are missing coverage. Please review.

Project coverage is 61.24%. Comparing base (29962e7) to head (c554ed1).
Report is 3 commits behind head on master.

Files	Patch %	Lines
examples/flattened_enum.rs	0.00%	64 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #738      +/-   ##
==========================================
- Coverage   61.48%   61.24%   -0.24%     
==========================================
  Files          38       39       +1     
  Lines       16213    16277      +64     
==========================================
+ Hits         9968     9969       +1     
- Misses       6245     6308      +63     

Flag	Coverage Δ
unittests	61.24% <0.00%> (-0.24%)	⬇️

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[Mingun]

Forget to mention: could you add an entry to the Changelog.md about new example, so people will aware that it is exist? Remember, that changelog contains section with changes and list of link targets at the end of each chapter (== version). This is standard markdown way to split link usage and link target.

All everything is good.

[Frankkkkk]

done. thanks

[Mingun]

Thanks! I slightly improved text in changelog