ERC-1271 contract signature verification for stETH

[skozin]

Allows stETH token to verify permit signatures issued by contracts. This is required to support account abstraction, among other things.

See:

• https://eips.ethereum.org/EIPS/eip-1271
• https://eip1271.io
• Uniswap v3 permit impl
• Balancer v2 permit impl