Make signature check method public (and view)

[rmeissner]

It should be possible to check the signatures against the default Safe access scheme.

Implementations:

• Remove storage clearing for approvedHashes and make existing function public view
• Move storage clearing out of the function (e.g. return owners that used approveHash) and create separate external view to check signatures from outside
  • As storage refunds might be deprecated soon anyways we should just remove the logic
  • https://eips.ethereum.org/EIPS/eip-3298
  • https://github.com/ethereum/EIPs/blob/9034dbdf1b526773e5210125fbadaa7ae4df8ace/EIPS/eip-3300.md