feat: Add WalletDb field to MempoolNode struct and initialize in constructor

[Sophie-Ag00001]

## Summary
This PR adds a `WalletDb` field to the `MempoolNode` struct and initializes it in the `MempoolNode::new()` function. No other existing code has been altered.

## Motivation
The addition of the `WalletDb` field is necessary to integrate wallet functionality into the `MempoolNode` struct, enabling it to interact with the wallet database. This change is part of a broader effort to enhance the node's capabilities by incorporating wallet-related operations.

## Context
The `MempoolNode` struct currently lacks the ability to interact with wallet data. By adding the `WalletDb` field, we enable the node to perform wallet-related tasks, such as querying balances or managing transactions, directly within the mempool context.

## Testing
To test the changes:
1. Ensure the `MempoolNode::new()` function correctly initializes the `WalletDb` field.
2. Verify that no existing functionality has been affected by the addition of the new field.
3. Run any relevant unit tests to confirm the integrity of the codebase.

## Known Issues
- None at this time. The changes are isolated to the addition and initialization of the `WalletDb` field, with no impact on existing code.