Support/Help Snowbridge design better UX fee model

[acatangiu]

Description

Current Snowbridge deployment:

1. users pay fees in source side native asset (DOT when coming from Polkadot, ETH when coming from Ethereum)
2. uses sovereign accounts both ways on the destination side for holding fees assets
3. has to manage/keep-up-to-date an onchain DOT/ETH exchange rate
4. is pretty expensive both ways

We want to improve UX (1 & 4) as well as decrease operational overhead (get rid of 2 & 3).

Ideas

• follow same pattern of Reducing bridge fees: short and long term options #2871
• users pay fees in ETH/WETH on both sides in both directions: on PAH it uses WETH/DOT pool for transparent conversion.
• with ^, we don't need SA anymore, for P->E direction auto-exchanged DOT is used on Polkadot side, and ETH is used on Ethereum side; for E->P direction we only need fees on destination AH where WETH/DOT pool auto-exchanges it. Relayers can be paid static overestimated fee - 0.1 DOT maybe (very cheap)
• no on-chain exchange rate required
• Ethereum -> Polkadot transfer fee is dirt cheap 😍

[acatangiu]

cc @vgeddes to add other discussed ideas