Added cognito PostAuthentication event struct

[mattrx]

Issue #, if available:

Description of changes:

Based on: https://docs.aws.amazon.com/cognito/latest/developerguide/user-pool-lambda-post-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 #190 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #190   +/-   ##
======================================
  Coverage    77.2%   77.2%           
======================================
  Files          18      18           
  Lines         636     636           
======================================
  Hits          491     491           
  Misses        104     104           
  Partials       41      41

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[bmoffatt]

Thanks!