feat: override safe nonce and verify signatures

[felipe-op]

This change adds a new environment variable called SAFE_NONCE which allow callers to override the Safe contract nonce.

This is useful when crafting future transactions ahead of time and collecting signatures.

It also provides a new function verify(bytes memory _signatures) to make sure the collected signatures are valid to execute the transaction.

It is intended to work as:

1. Maintainer craft a transaction ahead of time
2. Multiple signer parties are contacted and sign the transaction offline
3. Maintainer collect and verify the presigned transactions
4. Transaction with signatures are stored safely for later execution

[tynes]

uint256 nonce = vm.envOr("SAFE_NONCE", safe.nonce());