[services][blob] Add AWS SDK config

Summary:
Added a function to retrieve SDK config, required for accessing
the S3 and DynamoDB SDKs. It is unused yet, but will be needed
later.

Conditionally, if COMM_SERVICES_SANDBOX environment variable is set,
it uses localstack, which reflects the
current C++ implementation.

Depends on D5677

Test Plan: This function is tested a few diffs later, when S3 / DynamoDB are used.

Reviewers: varun, jon, tomek, marcin, ashoat

Reviewed By: varun, tomek, ashoat

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D5678