Implement support for s3 storage

[RodrigoPereyraDiaz]

• refactor to improve test and increment coverage

[codecov]

Codecov Report

Merging #175 (88e36c6) into master (cc3872a) will decrease coverage by 0.73%.
The diff coverage is 47.61%.

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
- Coverage   57.84%   57.10%   -0.74%     
==========================================
  Files          38       39       +1     
  Lines         389      408      +19     
==========================================
+ Hits          225      233       +8     
- Misses        164      175      +11     

Impacted Files	Coverage Δ
MailerQ/MessageStore/S3MessageStorage.cs	26.66% <26.66%> (ø)
MailerQ/MessageStore/MessageStorage.cs	100.00% <100.00%> (ø)
MailerQ/MessageStore/MongoDBMessageStorage.cs	54.54% <100.00%> (ø)

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