Notes from Design Review: Review also covered incentivized delivery in #272

General:

• all in favor of adding the context opaque bytes. We can do this ahead of time since it is an api breaking change with confidence due to it's general usefulness
• GasTanks are abstracted enough that can support payments for senders, msgHash, and more. (they can also refund the relayers on origin, destination or any other superchain).
• Not entirely sure about call depth in the propagated ctx, but we need a way to being able to:
  • reference the parent msg-hash on child sentMessages
  • We might be able to infer ordering of relayed messages on the GasTank via a chain of hashes with the parent message
• all in favor of adding the RelayedMessageGasReceipt event with minor modifications to be future-proof
• important we need to go through integration and dapp examples to see where this works and breaks.

Next Steps:

1. Update this document scoped to just propagating opaque context bytes with the SentMessage event. This will be empty to start
2. Update or create a new document for RelayedMessageGasReceipt which can be tied with feat: incentivized cross domain message delivery #272. When some prototypes of the gas tank is created, we can then make use of the context bytes with a first version (tx.origin, rootMsgHash) and emit the the gas receipt event.