Support signing in with Argent

[morleyzhi]

This would allow Gnosis safes and Argent wallets to sign in.

See also:

• https://eips.ethereum.org/EIPS/eip-1271
• https://github.com/authereum/is-valid-signature

[w4ll3]

Hello, siwe always supported EIP-1271 (All you need is to setup a provider). Is this still an issue for you? Also we are aware of some problems with Argent and WalletConnect V2, but that's not related to siwe.

[obstropolos]

Closing here.