[feature] Add getItemsCount() for the model

[rennokki]

Initial PR from @sahilsharma011
baopham/laravel-dynamodb#209

[codecov]

Codecov Report

Merging #4 into master will increase coverage by 0.07%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master       #4      +/-   ##
============================================
+ Coverage     85.62%   85.69%   +0.07%     
- Complexity      339      340       +1     
============================================
  Files            24       24              
  Lines           918      923       +5     
============================================
+ Hits            786      791       +5     
  Misses          132      132

Impacted Files	Coverage Δ	Complexity Δ
src/DynamoDbModel.php	80.68% <100%> (+0.68%)	62 <1> (+1)	⬆️

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