add TargetArn as valid destination for publish

[janjaali]

Addressed issue: #46

This PR adds targetArn besides to topicArn as additional valid destination-field to publish messages to the SNS.

[codecov-io]

Codecov Report

Merging #47 into master will increase coverage by 0.04%.
The diff coverage is 75%.

@@            Coverage Diff             @@
##           master      #47      +/-   ##
==========================================
+ Coverage   40.47%   40.52%   +0.04%     
==========================================
  Files          22       22              
  Lines         420      422       +2     
  Branches       13       13              
==========================================
+ Hits          170      171       +1     
- Misses        250      251       +1

Impacted Files	Coverage Δ
src/main/scala/me/snov/sns/api/PublishApi.scala	72.22% <75%> (-2.78%)	⬇️

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[s12v]

thank you, @janjaali