Snowbridge V2 - Rewards Pallet

[claravanstaden]

Description

Adds a rewards pallet, where rewards for relayers will be accumulated. More details in the Snowbridge v2 design: https://github.com/paritytech/polkadot-sdk/blob/master/bridges/snowbridge/docs/v2.md#relayer-rewards

Integration

Since this is a new feature, so it does not affect integration.

Review Notes

TODOs:
[ ] Add deposit from Inbound and Outbound queue.
[ ] Test XCM
[ ] More tests, including integration.

[bkontur]

We have a similar pallet here: https://github.com/paritytech/polkadot-sdk/tree/master/bridges/modules/relayers for registering relayers (deposits, slashing) and collecting rewards for relayer accounts.

I'm just curious if we could have a single generic pallet for relayers/rewards that could cover both the P/K bridge, the Eth bridge, or any other possible bridge. That would be cool. Essentially, we need to store the same things:

• Relayer account
• Reward identifier (for the P/K bridge, we use the kind of lane_id identifier for accumulating rewards; for ETH, I see it uses message_id)
• Reward (the P/K bridge uses the native token (DOT/KSM), while the ETH bridge uses WETH)

Some good abstraction over the reward identifier to support both, and using xcm::Asset for the reward could potentially cover everything.