[TASK] Implement leaves event watcher for protocol-ink! contracts

[dutterbutter]

Overview

We need to implement the vanchor events watcher similar to the normal anchor leaves watcher for protocol-ink!.

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

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