Explore possibility of using asset tx payment signed extension

Explore using pallet-asset-tx-payment in synergy with our tx fee payment storage to set default to simplify the logic of fee payment token usage.

We could remove the free transaction to set the fee payment token and keep only on account creation logic together with the signed extension which should reduce overall transaction cost and simplify the extensions we use while also providing wallets of setting this per transaction and removing the batch logic.

https://crates.io/crates/pallet-asset-tx-payment

Start by reviewing pallet-asset-tx-payment and the current transaction fee payment storage and signed extension logic. Determine whether the default fee token can replace the free setup transaction while retaining account-creation handling, per-transaction wallet selection, and no batch logic; document the resulting cost and behavior changes.

Written by the indexing model from the issue text.