test: Add integration tests to exercise the Lambda subscriptions

[ashovlin]

Issue #, if available: DOTNET-6922

Description of changes: Adds integration tests that create actual SQS queues mapped to Lambda functions.

Addressed feedback on #52 at the time it was closed (due to us stopping and then restarting this ticket now). I couldn't re-open since I rebased the branch on dev.

The majority of this is the scaffolding, right now the tests are just testing either a single successful or single failing message against ProcessLambdaEventAsync and ProcessLambdaEventWithBatchResponseAsync. But it's easier to iterate on tests now that the setup is in place, so open to feedback if you think we should add additional test cases.

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[96malhar]

Overall I am fine with the testing approach that we are taking. But I'd like to see 2 additional logical pieces being exercised (should be easy to add since the AWSUtilities and TestUtilities does most of the heavy lifting)

• Verify that the message source is correctly resolved from AWS_LAMBDA_FUNCTION_NAME environment variable
  • Log the message source to cloudwatch and assert on it.
• Verify FIFO processing of messages
  • Process 3 or more messages (belonging to the same group) from a FIFO queue and log their ProcessEnd times to CloudWatch. Assert that they were processed in the same order in which they were published.

[ashovlin]

Overall I am fine with the testing approach that we are taking. But I'd like to see 2 additional logical pieces being exercised (should be easy to add since the AWSUtilities and TestUtilities does most of the heavy lifting)

* Verify that the message source is correctly resolved from `AWS_LAMBDA_FUNCTION_NAME` environment variable
  
  * Log the message source to cloudwatch and assert on it.

* Verify FIFO processing of messages
  
  * Process 3 or more messages (belonging to the same group)  from a FIFO queue and log their `ProcessEnd` times to CloudWatch. Assert that they were processed in the same order in which they were published.

Added FIFO tests in c416d61.

I started going down the path of testing the message source, but currently we're not actually sending any messages from the Lambda function. I started a "bidirectional" test where we'd message the Lambda and then it'd reply with a forwarded message that we can assert the source on the client. But we'd have to expand the IAM permissions given to the Lambda to allow publishing (with the queueURL), as well as run a poller on the client. Overall it was feeling like more effort and setup code/time than it was worth.