[ICS04] Expose channel message types as public

[Farhad-Shabani]

Overview

Currently, our domain message types in the ics04_channel module are defined as private - pub(crate) - restricting external libraries from accessing and utilizing them. (At the moment, only these domain message types are not publicly accessible).
This came to my attention when splitting our mock implementation into a separate crate. In order to craft IBC messages for various testing scenarios, the mock implementation requires access to these domain types.

Proposal

Making the ics04_channel domain message types public to enable other libraries to access and use them as necessary.