Separate mock module into a standalone crate

[Farhad-Shabani]

Overview

As part of our efforts to make ibc-rs more streamlined, enhance usability, and easier to maintain, we are starting by separating the mock module into its own crate. This is a prerequisite step before refactoring our mock testing facilities, which ultimately will improve our testing coverage for ibc-rs and would simplify writing different testing scenarios by host chains.

Proposal

Create a separate crate for the mock module to achieve the following objectives:

1. Decouple the ibc-rs implementation from testing types and components.
2. Facilitate refactoring mock implementation so that can have more organized/comprehensive integration tests
3. Avoid circular dependencies, so would allow importing basecoin-rs libraries.
4. Make it easy to use the Merkle tree implementation within basecoin-store
5. Advance the goal of breaking down ibc-rs into smaller, self-contained and modular libraries