Added AuthenticationRegion to AmazonS3Client Endpoint based clients

[mdupras]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

Simply adding the optional AuthenticationRegion property to the AmazonS3Config.

#262

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #263 (f8f5047) into main (b1c5e2f) will increase coverage by 0%.
The diff coverage is 100%.

@@         Coverage Diff         @@
##           main   #263   +/-   ##
===================================
  Coverage    85%    85%           
===================================
  Files        75     75           
  Lines      2036   2036           
  Branches    298    298           
===================================
+ Hits       1739   1741    +2     
+ Misses      213    211    -2     
  Partials     84     84           

Flag	Coverage Δ
unittests	85% <100%> (+<1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...geSharp.Web.Providers.AWS/AmazonS3ClientFactory.cs	18% <100%> (ø)
.../Synchronization/RefCountedConcurrentDictionary.cs	76% <0%> (+4%)	⬆️

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[JimBobSquarePants]

LGTM 👍 Thanks!