chore(code): Split `HostMsg` out of `engine` crate and into a new `host` crate

[romac]

Now that https://github.com/informalsystems/malachite/pull/1109 is merged, we should be able to split the HostMsg enum out of the malachitebft-engine crate and into its own malachitebft-host crate. We can then re-export it from the malachitebft-engine crate to avoid introducing a breaking change.

[spoo-bar]

Hey, may I work on this task as its unassigned?

[Ridwannurudeen]

Hi, I'd like to take this on. I see there's an open PR (#1252) from November that appears to be stale — happy to start fresh if that's okay with the team.

My plan:

1. Create a new malachitebft-host crate under code/crates/
2. Move the HostMsg enum (and any tightly coupled types) from malachitebft-engine into it
3. Re-export HostMsg from malachitebft-engine to maintain backward compatibility
4. Update internal imports across the workspace
5. Ensure all existing tests pass

Could you /assign this to me? Thanks!

[romac]

@Ridwannurudeen Thank you very much for your interest, but there is no need for another implementation of this. Thanks also for the reminder that I need to get to that PR :)