Add after_find callback

[andrykonchin]

Changes:

• added after_find callback
• added specs on both after_initialize and after_find callbacks

after_find callback works in the same way like it does in Rails. It's called after after_initialize at loading a model from DynamoDB.

The following methods trigger it:

• find
• find_all
• reload
• where
• last/first
• find_by_page

https://guides.rubyonrails.org/active_record_callbacks.html#after-initialize-and-after-find

[codecov[bot]]

Codecov Report

Merging #645 (c425697) into master (47d9492) will increase coverage by 0.01%. The diff coverage is 100.00%.

:exclamation: Current head c425697 differs from pull request most recent head 9806467. Consider uploading reports for the commit 9806467 to get more accurate results

@@            Coverage Diff             @@
##           master     #645      +/-   ##
==========================================
+ Coverage   90.27%   90.28%   +0.01%     
==========================================
  Files          61       61              
  Lines        3136     3141       +5     
==========================================
+ Hits         2831     2836       +5     
  Misses        305      305              

Impacted Files	Coverage Δ
lib/dynamoid/components.rb	100.00% <100.00%> (ø)
lib/dynamoid/criteria/chain.rb	99.58% <100.00%> (+<0.01%)	:arrow_up:
lib/dynamoid/finders.rb	94.84% <100.00%> (+0.22%)	:arrow_up:

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[github-actions[bot]]

Package	Line Rate	Health
dynamoid	90%	➖
Summary	90% (2836 / 3141)	➖

Minimum allowed line rate is 90%