Update SigV4 sample code

[jkielbaey]

*Issue #327 327

Description of changes:

Updated the sample code for the Lambda function to process the trigger from S3 and invoke Workflow API. The updates include retrieving configuration details (such as region) from the environment variables and the Lambda event (S3 bucket name), as well as the ability to use short lived credentials associated with the IAM role used by the Lambda function.

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