As a developer, I want callbacks on origin domain to be processed by relayers

[just-a-node]

Problem Callback flow:

• xcall
• execute -> this sends message back to callback via nomad
• PromiseRouter.handle -> registers data for callback
• PromiseRouter.callback -> executes the callback function

Right now we're freezing up at the last step (nomad message returned to origin chain, nothing processing the transported message). We cant put arbitrary gas consumption onto the nomad handler, which is why we have a second processing step here.

Ideas to solve this We need to relayers to act on origin domain events (most likely BridgeRouter's Receive event) to process the callback execution.

Acceptance Criteria [ ] Relayers watch for handled events and process messaging

[alexwhte]

@just-a-node Do any major integrations need this ASAP for the initial launch? ruthlessly prioritizing the issues

[just-a-node]

@just-a-node Do any major integrations need this ASAP for the initial launch? ruthlessly prioritizing the issues

Not for initial launch - if they need callbacks executed they can specify their own EOA to call it.

[alexwhte]

closing in favor of nested xcalls