Move pallet-asset-tx-payment into Substrate

[apopiak]

There is nothing cumulus specific about pallet-asset-tx-payment so it makes sense to upstream it to the Substrate repo.
As it wraps and extends pallet-transaction-payment it makes sense for it to live there.

[nuke-web3]

This will be a minor breaking change on the next release, right? Deps will need to be updated in Cargo (only) - anything else that it might imply?

[georgesdib]

I have taken a stab at this in paritytech/substrate#10127
The corresponding cumulus PR is #712 but, correct me if I am wrong, I will need to update Cargo.lock only after the substrate PR is merged.

[apopiak]

@georgesdib You will need to set the dependency of #712 to your substrate branch and communicate to the bot that it is the companion for paritytech/substrate#10127