[Hardhat] Convert `getHtsStorageAt` into a method to support persistent state

PR https://github.com/hashgraph/hedera-forking/pull/173 introduces a stateful mechanism to deal with `tokenURI` occupying more than one slot. This state is global, which lacks flexibility and could cause issues and confusion (given the state is not explicit) for consumers of the package.

To solve this problem, we should convert the `getHtsStorageAt` function into a class method and move the persistent state into the new class. This way, the state is now explicit, avoiding any surprise to the user.