Add clientMetadata to triggers that support it

[thepelkus-too]

Issue #, if available:

#255

Description of changes:

Request structs for triggers are updated with a ClientMetadata field when supported as noted in the documentation under https://docs.aws.amazon.com/cognito/latest/developerguide/cognito-user-identity-pools-working-with-aws-lambda-triggers.html

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

[bmoffatt]

AWS CodeBuild CI Report

• CodeBuild project: aws-lambda-go-github-ci
• Commit ID: db17044
• Result: SUCCEEDED
• Build Logs (available for 30 days)

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[codecov-io]

Codecov Report

Merging #256 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #256   +/-   ##
=======================================
  Coverage   74.59%   74.59%           
=======================================
  Files          20       20           
  Lines         681      681           
=======================================
  Hits          508      508           
  Misses        128      128           
  Partials       45       45

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[bmoffatt]

AWS CodeBuild CI Report

• CodeBuild project: aws-lambda-go-github-ci
• Commit ID: 6b4e9f4
• Result: SUCCEEDED
• Build Logs (available for 30 days)

Powered by github-codebuild-logs, available on the AWS Serverless Application Repository

[bmoffatt]

LGTM, thanks!