Lambda basics scenario example for Swift

[shepazon]

This pull request contains a complete implementation of the "basics" scenario example for the AWS SDK for Swift. To use it:

1. Pull the package dependencies (swift package update) to be sure the Lambda runtime module is available.
2. Build the increment lambda function (cd increment && swift build).
3. Archive the increment lambda function for use as a Lambda function: swift package archive --allow-network-connections docker. Take note of the Zip file's path when it's displayed once archiving is complete.
4. Build the calculator lambda function (cd ../calculator && swift build).
5. Archive it for use as a Lambda function: swift package archive --allow-network-connections docker. Take note of the created ZIp file's path.
6. Build the main program in the lambda-basics directory.
7. Run the main program with the command swift run lambda-basics --incpath <path-of-increment-archive> --calcpath <path-of-calculator-archive>.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.