Add events for Cognito custom authentication challenge

[mxpv]

This PR adds events for Cognito's custom authentication - https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-challenge.html

[codecov-io]

Codecov Report

Merging #168 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #168   +/-   ##
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  Coverage    77.2%   77.2%           
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  Files          18      18           
  Lines         636     636           
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  Hits          491     491           
  Misses        104     104           
  Partials       41      41

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[bmoffatt]

Not sure that CognitoEventUserPoolsVerifyAuthChallengeRequest.ChallengeAnswer is the right type after reading the docs

[hgl]

Any chance this gets merged anytime soon?

[mxpv]

@bmoffatt I'm not quite sure whether map[string]string is the right choice since on my test env I was receiving a string type value, so I decided to go with interface{}

[mxpv]

ping @bmoffatt

[bmoffatt]

@bmoffatt I'm not quite sure whether map[string]string is the right choice since on my test env I was receiving a string type value, so I decided to go with interface{}

👍