Optional Clientv2 API

[loganb]

This adds a new adapter that uses V2 DynamoDB Client API. In the future, this should make it possible to add secondary index support (though this PR doesn't include it) as well as enable the use of newer versions of fake_dynamo.

I implemented as-needed until all specs passed, and I endeavored to make zero non-adapter changes, effectively keeping the interface to the upper layers of Dynamoid fixed. Thus, it is likely that the functionality is incomplete and/or lacking performance. Additionally, many sections of code are quite ugly, so once Dynamo is migrated to this interface, there will need to be successive rounds of cleanup where code is simplified by tweaking the adapter interface to the upper layers of software.

I'll continue to clean up the code a bit, and eventually merge it as an optional adapter before making it permanent.

[kris-evans]

Is there any word on when this will be merged?