[TASK] Relayer Incentives

[akileshtangella]

• Native token is a scarce resource. Submitting proposals to the SignatureBridge depletes a relayer's native token stash due to gas costs.
• Therefore, we want to reimburse the relayer for its gas costs.
• Initial idea: for native token (i.e. ETH) pools, the feeRecipient is the SignatureBridge and upon a succesful proposal submission the relayer is transferred the native token.
• Further iteration: Anyone can call a function to swap unwrapped token to native asset and transfer to relayer.
• Integrate with GSN: can pay relayer directly with ERC20 tokens.