Extending asset-tx-payment with default per signed origin

[dzmitry-lahoda]

I want to extend
https://github.com/paritytech/substrate/blob/master/frame/transaction-payment/asset-tx-payment/src/lib.rs

with ability to set default asset for user.

So that,

1. signed origin can call extrinsic with desired asset id
2. it will call on charge asset transaction with fee equal for relevant weight for TX
3. ED for storage of default will be captured in target asset
4. when user sets new asset or resets to none, ED is returned to him.
5. when user set new asses, he pays with new asset for TX and taken ED in new asset.
6. when user sets new asset to none, I will try to pay in native assets, if fails, take part of ED and set to none - so we are glad do use native asset
7. Either user or root can set the value. Root does not pays.

Is it ok?

What will happen if asset will be removed from payment?
User will get errors and should be able to set to none his asset or to new asset.