Feature Request: support ReturnValuesOnConditionCheckFailure

[extemporalgenome]

The write operations which support ReturnValues (PutItem, UpdateItem, DeleteItem, transactional equivalents thereof, and possibly others) generally also support ReturnValuesOnConditionCheckFailure, which can take a value of ALL_OLD to get a strongly consistent read for free (in terms of capacity cost) even if the write was semantically rejected.

This is pretty useful from an atomics perspective, i.e. inspecting the item in a guaranteed state to find out why the condition check failed, all without needing to perform a potentially racing followup read.

Here is a non-complete list of the operations that support this (at least the ones I care most about), and the relevant docs/code:

Operation	Docs	SDK V1	SDK V2
PutItem	link	link	link
UpdateItem	link	link	link
DeleteItem	link	link	link

[guregu]

Oh nice, I didn't know this existed (is it new?). I will add it for sure.
Edit: oops I had a TODO for this (well, the tx version) I totally forgot about because the SDK didn't support it yet. Working on it now.

[guregu]

PR here: #246. Wasn't sure whether it was better to have an API that works based off the errors or another method on the operations so I did both :-)
Let me know if it looks good.

[guregu]

Just released in v2.2.0. Let me know if you run into any issues

[extemporalgenome]

Thanks!