Please add support for reading the AWS_REGION environment variable as well

Currently, the code only looks for SERVICE_REGION and fails if it is not found, in AWS Lambda, the environment variable 'AWS_REGION' is prepopulated by default and can be used.

https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars.html

Start by locating where SERVICE_REGION is read and inspect any existing tests for region configuration. The work is done when AWS_REGION is accepted in AWS Lambda while existing SERVICE_REGION behavior remains covered.

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