dynamodb: add alternative to Integer conversion helper, Int64, that does not silently cast float values

[bmoffatt]

Issue #, if available:

#454

Description of changes:

Same as Integer(), but without the "convenience" of falling back to float parsing.

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[codecov-commenter]

Codecov Report

Merging #459 (46ca97c) into main (6796528) will increase coverage by 0.07%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #459      +/-   ##
==========================================
+ Coverage   72.90%   72.97%   +0.07%     
==========================================
  Files          19       19              
  Lines        1085     1088       +3     
==========================================
+ Hits          791      794       +3     
  Misses        225      225              
  Partials       69       69              

Impacted Files	Coverage Δ
events/attributevalue.go	78.46% <100.00%> (+0.20%)	⬆️

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