Add Amazon MSK event structure

[ewbankkit]

Closes #315.

Adds Amazon MSK event structures.

$ go test ./events/
ok  	github.com/aws/aws-lambda-go/events	0.020s

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[codecov-commenter]

Codecov Report

Merging #316 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #316   +/-   ##
=======================================
  Coverage   72.23%   72.23%           
=======================================
  Files          18       18           
  Lines         724      724           
=======================================
  Hits          523      523           
  Misses        136      136           
  Partials       65       65           

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[bmoffatt]

@carlzogh should we name this KafaEvent instead to match the java library?

ref: https://github.com/aws/aws-lambda-java-libs/blob/master/aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/KafkaEvent.java

[carlzogh]

@carlzogh should we name this KafaEvent instead to match the java library?

ref: https://github.com/aws/aws-lambda-java-libs/blob/master/aws-lambda-java-events/src/main/java/com/amazonaws/services/lambda/runtime/events/KafkaEvent.java

Yes I think we should name it KafkaEvent to stay consistent.

[ewbankkit]

Renaming done.