Implement statically typed output types

[simar7]

Currently there are no statically defined return outputs for every Store implementation as there are for inputs.

So this would possible look something like the following:

type Store interface {
	Set(input types.SetItemInput) (types.SetItemOutput, error)
	BatchSet(input types.BatchSetItemInput) (types.BatchSetItemOutput, error)
	Get(input types.GetItemInput) (types.GetItemOutput, error)
	Delete(input types.DeleteItemInput) (types.DeleteItemOutput, error)
	Close() error
	Scan(input types.ScanInput) (types.ScanOutput, error)
}