`ConditionCheck` transaction operation support

[agostbiro]

We currently support Put, Update and Delete operations for Table.transact_write_items, but not ConditionCheck.

From the DynamoDB docs:

ConditionCheck — Applies a condition to an item that is not being modified by the transaction. This structure specifies the primary key of the item to be checked, the name of the table where it resides, a condition expression that must be satisfied for the transaction to succeed, and a field indicating whether to retrieve the item's attributes if the condition is not met.

Please give this issue a thumbs up if you need ConditionCheck support for Table.transact_write_items.