Proposal: Deprecate Message Relayer service in favor of SDK

[smartcontracts]

I think the Message Relayer service could be replaced by the SDK. The Message Relayer is an optional service added that can be added to the Optimism local development ("ops") setup. We do not use this service in CI and we don't have any tests for the service. I also believe that the Message Relayer creates an unrealistic expectation of the behavior of Optimism for developers. Most application developers who want to include some sort of cross-chain message will need to ask their users to explicitly finalize L2 to L1 messages. I really see no clear benefit in keeping the package and the SDK will be good enough that it should be really easy for developers to handle withdrawals themselves in their tests.

Removing the service would require the following:

• Have the SDK go to production so that we can be sure the API is clean enough for developers to use in their tests. (2-4 week beta period)
• Add docs to show how to use the SDK to relay messages (1-2 days).
• Create a deprecation notice and distribute the notice publicly (4 weeks).
• Remove the package and officially drop support for the service.

Please let me know if you have strong feelings about keeping the Message Relayer.

[smartcontracts]

@palango brings up that the package is useful in non-typescript environments: #2179 (comment)

[smartcontracts]

We've decided not to deprecate the message relayer because of it's utility in non-typescript environments.