Build tax content file for location async

[joshwaldrep]

Fixes the incorrect deserialization of the BuildTaxContentFileForLocationAsync method. The response is an array of tax locations, item codes, and rates. The SDK is trying to deserialize it into a FileResult Object. The call always fails on the deserialization.

I couldn't find a class that matched the return values of the API so I added one. If there is an existing class, that would probably be better. I had to assume the data types based on the values returned by calling the API directly and looking through the other model classes.

I don't know Avalara's documentation standards so there will probably be some updates needed before this is merged.

I didn't find anything in the tests project that references this method.

[joshwaldrep]

There's an open issue for this, but I don't have access to link it. Invalid response type BuildTaxContentFile(ForLocation)(Async) #145