Implement VaultManager using EIP4626 for better UX

[0x-r4bbit]

Users can stake only once per StakeVault.
This means, if they stake multiple times, they will create multiple StakeVaults.
To make managing and handling those vaults easier, it's worth considering abstracting all create vaults behind a single API.

This can possibly be done with EIP4626 as it's an EIP designed for tokenized vaults.

The goal of this issue is to create a first functioning version of such a vault manager. Including some tests that demo its functionality.

One thing we should also think about is how it could help with executeAccount() on individual vaults.