Make `is_promotable` helper function a proper extended API command

Current helper method only checks for consistency, but it should also verify that `attachmentTimestamp` is above a lower bound that is calculated based on number of milestones since the transaction attachment.

Reference from [iota.js](https://github.com/iotaledger/iota.js/blob/next/api_reference.md#module_core.isPromotable).

Make a proper extended API command out of the helper method.