Add Generated annotation to generated classes

[pkoenig10]

This annotation is used to inform other tools that this code is generated.

For example, error-prone uses this annotation to suppress warnings in generated code when the XepDisableWarningsInGeneratedCode flag is set.

[codecov-io]

Codecov Report

Merging #94 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #94      +/-   ##
============================================
+ Coverage     89.96%   89.99%   +0.03%     
- Complexity      849      851       +2     
============================================
  Files            56       56              
  Lines          2949     2958       +9     
  Branches        136      136              
============================================
+ Hits           2653     2662       +9     
  Misses          235      235              
  Partials         61       61

Impacted Files	Coverage Δ	Complexity Δ
...java/org/derive4j/processor/DerivingProcessor.java	72.86% <100%> (+1.55%)	34 <1> (+1)	⬆️
...n/java/org/derive4j/processor/DeriveUtilsImpl.java	87.65% <100%> (+0.05%)	168 <2> (+1)	⬆️

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