Fix #1035 - Add apiKeyId to identity information

[jeroenvollenbrock]

Description

Adds the ApiKeyId to the identity object in the requestContext of the LambdaProxyIntegrationEvent

Motivation and Context

When testing locally it sometimes is needed to read out the apiKeyId instead of the apiKey
#1035

How Has This Been Tested?

I npm link'ed this branch in my local environment and verified the apiKeyId was available in the event.

There seems to be no unit test that covers the contents the identity object, except for a file in the folder called "old-unit". Please let me know if adding an additional expect there is appreciated.

Screenshots (if appropriate):

[frozenbonito]

Thank you @jeroenvollenbrock.
Would you be able to add the environment variable SLS_API_KEY_ID to the documentation?
https://github.com/dherault/serverless-offline#environment-variables

[jeroenvollenbrock]

Done!

[jeroenvollenbrock]

@frozenbonito The three failed CI tasks seem unrelated, they might need to be retriggered