Adding returnedMessages primitive into L2ToL2CrossDomainMessenger

[skeletor-spaceman]

Description

Adds a way for the L2ToL2CrossDomainMessenger to "send-back" failed messages.
These failed messages are stored on the origin domain's L2ToL2CrossDomainMessenger.
This enables smart contracts to handle "failed" messages.
e.g. a future L2toL2StandardBridge will be able to safely recover stuck funds by validating that a message was indeed NOT successful on the other domain.

For this proposal, we've also prepared a detailed Notion document, found here, that goes through the rationale and need of the returnedMessages mapping and accompanying functionality and required modifications.
We've also prepared an example of the technical PR

We believe this would be an important primitive for Interop and the canonical bridge.

Would love to hear your thoughts and any feedback you might have on this proposal :)

Tests

N/A

Additional context

https://defi-wonderland.notion.site/L2-L2-Stuck-Messages-71a4cdb5a06c48fb94d0b98d89c46a03
previous PR: #184

Metadata

N/A

This proposal was developed as an internal research project at @defi-wonderland lead by @0xng.

[tynes]

Just to be clear that the StandardBridge and the L2ToL2CrossDomainMessenger are in different bridge systems. The L1<>L2 bridging contracts are strictly different than the L2<>L2 bridging contracts, so a change to the L2ToL2CrossDomainMessenger wouldn't impact the L1<>L2 StandardBridge flow

[skeletor-spaceman]

Just to be clear that the StandardBridge and the L2ToL2CrossDomainMessenger are in different bridge systems.

oh nice catch, since we don't yet have a L2toL2StandardBridge, I mentioned it as an example, but I'll add a clarification as to not get things confused with the L1<>L2 flow.

[tynes]

I am generally in favor of this approach and would be open to getting it merged, I think it can be a powerful primitive. It doesn't solve the problem of sending to a chain not in the dependency set but that might be impossible to solve (?)

Left a set of comments that are generally minor/aesthetic