Add basestring to allowed types for Code property of Function class

[haylix-edward]

In order to use the "aws package" command, we need to be able to provide a file/folder path to the Function resource as a simple string value.

[JordonPhillips]

@haylix-edward I believe what you're looking for is aws.serverless.Function (aws docs).

[haylix-edward]

@JordonPhillips We did look at using those, but they don't work inside nested cloudformation stacks (it has been long enough that I don't recall exactly what the issue was).