[TASK] Implement leaves event watcher for Cosmwasm

[dutterbutter]

Overview

We need to implement the vanchor events watcher similar to the normal anchor leaves watcher for Cosmwasm.

A good example of the anchor pallet leaves watcher is here.

• [ ] Implement Cosmwasm event watcher trait for Transaction event from VAnchor pallet.
• [ ] Save the leaves inside the store
• [ ] Add applicable tests and documentation

[dutterbutter]

Related to: https://github.com/webb-tools/relayer/pull/263