Do not require "digest" dynamo column

[travisgosselin]

The digest column is not currently utilized for anything in retrieval. I have an implementation of credstash and I need a .net core client for but with the use of a required "digest" column in the schema.

I also added the explicit SDK version to avoid any runtime conflicts with newer .net core preview versions.

[adamhathcock]

Thanks!

I'll merge this tomorrow UK morning and get it on nuget.

I'll see if any other columns could be optional.

[travisgosselin]

Awesome thanks @adamhathcock