Define the commerce payment transaction entity type.

Here is a first draft of the commerce_payment_transaction entity type shipped with the commerce_payment module. No forms are defined so far, only database model has been implemented.

Please feel free to blame me :)
I don't believe we need an instance ID any more, or at least we won't call it the same. This property in Commerce 1.x was just a hash to locate the rule that enabled a payment method so the payment method configuration could be extracted from its action. In Commerce 2.x, we'll be storing payment method configurations via some entity type (it isn't clear to me if it will be a content entity type or configuration entity type). We can change this from instance_id to payment_method_id at the very least; but we can always make this change after we've defined the payment method entity type. : )