fix: skip adding authorizer to event if no authorizer is configured

[cnuss]

Description

AWS API Gateway does not include the event.requestContext.authorizer field if there is no authorizer configured for the function.

This PR deletes the event.requestContext.authorizer field if the function does not have an authorizer.

Motivation and Context

• I'm doing serialization of API Gateway in Golang
  • It was failing because the authorizer payload didn't match what it expected
• Upon further investigation, I found that serverless-offline was always setting event.requestContext.authorizer if a Bearer token comes through in the HTTP Authorization headers.
• My solution is to simply remove event.requestContext.authorizer if the lambda function does not have an authorizer set.

How Has This Been Tested?

I've written tests and included them in this PR. Please let me know if you want me to kick the tires in any other ways!

[cnuss]

Hi @DorianMazur i have another one for you to 👀 !

[cnuss]

thank you! i ran a limited set of the tests locally until it worked so i must have missed customAuthenticationProvider