Config Schema: Fix SNS event schema (allow both arn and topicName in sns event)

[medikoo]

Fixes regression introduced with #8323

There are valid scenarios where arn and topicName should be provided

[codecov-commenter]

Codecov Report

Merging #8331 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #8331   +/-   ##
=======================================
  Coverage   88.01%   88.01%           
=======================================
  Files         249      249           
  Lines        9307     9307           
=======================================
  Hits         8192     8192           
  Misses       1115     1115           

Impacted Files	Coverage Δ
...ib/plugins/aws/package/compile/events/sns/index.js	100.00% <ø> (ø)

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