Warp Routes should allow LPs to front the liquidity

[nambrot]

Warp Routes are secured by ISMs, i.e collateral that is stored on let's say HypERC20Collateral is only redeemable if the configured ISM attests to the corresponding instruction from the permissioned contract on the remote chain. When using a relatively slow ISM (like an Optimistic ISM or an underlying canonical optimistic rollup bridge), the latency in transferring via a warp route could be not ideal. This bounty proposes a proof of concept that allow users to specify an optional fee to have LPs provide the underlying token before the ISM "finalizes". To implement this, you would do the following:

• Create variants of the existing Warp route contracts
• On the remote chain, allow warp route transfers to embed "fast metadata" to indicate a user's willingness to pay an LP. For simplicity, start with a single parameter fastFee that can be called via a transferRemote variant
• On the local chain, allow anybody (LP) to call fillFastTransfer(recipient, amount, fastFee) which takes the tokens (amount - fastFee) from msg.sender and gives them to the recipient. Store evidence of the action in storage.
• Upon handle of the message, check that fastFee was set in the metadata, check if somebody has indeed filled the transfer previously, and if so, actually transfer the tokens to the LP. Otherwise, completely the transfer as normally.

To close out this ticket, create unit tests with the mocks that illustrate this behavior

[RohanShrothrium]

@nambrot I will work on this!