post_dispatch hook for SignedExtension

[xlc]

Currently there is a pre_dispatch hook to do additional work before transaction is dispatched. It will be useful to have a post_dispatch hook to do additional work after transaction is successfully dispatched.
My use case is to implement a fees module that emit fees.Charged(Amount) event to notify how much transaction fee is been charged in this transaction. This have to be in post_dispatch because modules may charge additional fee (balances create account) or refund fee (contracts refund gas fee).