Consider adding a totalStaked per address getter

The contract provides getPositionIdsForAddress to retrieve all position IDs for a wallet, but lacks a convenient way to get the total amount staked across all positions for a specific user. Currently, to get this information, a client would need to:

1. Call getPositionIdsForAddress.
2. Iterate through each ID.
3. Call getPositionById for each position.
4. Sum up the tokensStaked values.

It might be useful to add a getTotalStakedForAddress() function.