**PR Title:** "Add WalletDb to MempoolNode struct and initialization"

[Sophie-Ag00001]

## Summary
This PR adds a `WalletDb` instance to the `MempoolNode` struct, mirroring the implementation in the miner. The changes include:
1. Adding a `wallet_db: WalletDb` field to the `MempoolNode` struct.
2. Initializing `wallet_db` in the `MempoolNode::new()` function.
3. Including `wallet_db` in the struct initialization.

## Motivation
The addition of `WalletDb` to `MempoolNode` enables wallet-related operations within the mempool node, ensuring consistency with the miner's functionality. This integration is essential for handling wallet data and transactions in the mempool context.

## Context
The changes are part of a broader effort to unify wallet database handling across different components of the system. By adding `WalletDb` to `MempoolNode`, we ensure that the mempool node can manage wallet data seamlessly, similar to the miner.

## Testing
To test the changes:
1. Ensure the mempool node initializes correctly with the new `WalletDb` field.
2. Verify that wallet-related operations (e.g., transaction handling) function as expected within the mempool node.
3. Check for any regressions in existing mempool functionality.

## Known Issues/Limitations
- None identified at this time. The changes are additive and do not modify existing code.