Represent GRG Staking Proxy balances as ERC4626

[gabririgo]

Summary

Represent GRG staking proxy user balances as ERC4626 to allow easier and better data display by service providers.

Motivation

External applications now cannot see a wallet's staked GRG balance (vault balance plus unclaimed rewards). This requires developing a custom extension to service providers like DefiLlama or Zapper.
ERC4626 provides a standard for vaults, which exposes methods that return the underlying token balance. By implementing this standard, external service providers could display staked GRG in a user's wallet and their $ value. This would be particularly useful for Etherscan display, where it is not possible to develop a custom smart contract position fetcher.

Specification

• use OpenZeppelin ERC4626 standard;
• import interface methods;
• use their implementation;
• do not implement transfer, transferFrom and approve methods as GRG staked positions are non-transferable.

Notice: assert ERC-4626 inflation attack cannot be executed.

Notes

OpenZeppelin reference code
ERC-4626 inflation attacks