Add Money.with_bank for Dynamic Currency Stores

[pranavbabu]

This PR introduces the Money.with_bank method, allowing developers to assign different currency stores (banks) dynamically within a scoped block. This is particularly useful for loading custom exchange rates per user session.

Key Changes

• Implemented Money.with_bank(bank_instance), enabling temporary overrides of the default bank.
• Ensures thread safety by restoring the original bank after execution.
• Added documentation and an example demonstrating its usage in a Rails controller with around_action.