Add missing 'apiKeyId' to APIGatewayRequestIdentity

[nhibberd]

Description of changes: Adding missing apiKeyId to APIGatewayRequestIdentity

Similar issue / changes - aws/serverless-java-container#136
Some documentation that makes reference to apiKeyId - api-gateway-mapping-template-reference

Also happy to add anything that's required to get this through

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[codecov-io]

Codecov Report

Merging #235 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #235   +/-   ##
=======================================
  Coverage   74.44%   74.44%           
=======================================
  Files          20       20           
  Lines         677      677           
=======================================
  Hits          504      504           
  Misses        128      128           
  Partials       45       45

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[bmoffatt]

Thanks for the fix!