Add event types for Cognito PreAuthentication Lambda trigger

[markneves]

Adds the following Cognito event types:

• events.CognitoEventUserPoolsPreAuthentication
• events.CognitoEventUserPoolsPreAuthenticationRequest
• events.CognitoEventUserPoolsPreAuthenticationResponse

Added tests are based on existing Cognito tests sourced from this repository (specifically: TestCognitoEventUserPoolsPreSignupMarshaling and TestCognitoUserPoolsPreSignupMarshalingMalformedJson).

Parameters conform to AWS documentation for Cognito PreAuthentication: https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-pre-authentication.html

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-io]

Codecov Report

Merging #214 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
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  Coverage   75.88%   75.88%           
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  Files          18       18           
  Lines         647      647           
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  Hits          491      491           
  Misses        115      115           
  Partials       41       41

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[bmoffatt]

Thanks!