Add lex response

[ezhdan-nyt]

Issue #196

Add missed LexResponse struct, update events README

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[codecov-io]

Codecov Report

Merging #197 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #197   +/-   ##
======================================
  Coverage    77.2%   77.2%           
======================================
  Files          18      18           
  Lines         636     636           
======================================
  Hits          491     491           
  Misses        104     104           
  Partials       41      41

Impacted Files	Coverage Δ
events/lex.go	0% <ø> (ø)	⬆️

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[bmoffatt]

matches https://docs.aws.amazon.com/lex/latest/dg/lambda-input-response-format.html#using-lambda-response-format

Thanks for the fix!