Cleanup `OutboxMessage` abstraction

[slinkydeveloper]

The OutboxMessage is simply a subset of Command, it's an abstraction we don't really need and right now it's even using different serialization formats, potentially causing issues and requiring more code to be written. We should at least align the serialization of the two, or just plain remove OutboxMessage as it's not really useful to be a separate abstraction.

[slinkydeveloper]

See https://restatedev.slack.com/archives/C063T6B7S7M/p1736754461653159

[muhamadazmy]

I will try to think about the possibility of migrating to bilrost as well while working on this.