Upgrade to AWS SDK v4

[lailabougria]

This pull request is currently blocked due to a policy deserialization issue in the AWS SDK that should be fixed soon

What is left to do:

• Update to the SDK with the bug fix
  
• Figure out the clock skew code and whether it requires caching. It would also be possible to switch to something like

var serviceOperationEndpoint = sqsClient.Config.DetermineServiceOperationEndpoint(new ServiceOperationEndpointParameters(new GetCallerIdentityRequest()));
                var clockSkew = CorrectClockSkew.GetClockCorrectionForEndpoint(serviceOperationEndpoint.URL);

• Do further checks on the code path to figure out whether all the collection types have appropriate null checks.