sapphire-contracts: Add EIP-4361 SIWE parser and base contract

[matevz]

Fixes #327

This PR:

• adds contracts/SiweParser.sol, an on-chain parser for SIWE messages
• adds contracts/auth/A13e.sol, an abstract interface for authenticated queries (an alternative to signed queries which will get deprecated in https://github.com/oasisprotocol/sapphire-paratime/pull/303)
• adds contracts/auth/SiweAuth.sol, an on-chain authenticator for SIWE.
• tests, natspec docs + example

TODO:

• [ ] ~make SiweParser self-contained (merge DateTime into SiweParser?)~ (this doesn't actually improve anything, just adds clutter)
• [x] use error constants in SiweParser and SiweAuth

Before (signed queries)

contract MyContract {
  address private _owner;

  modifier onlyOwner() {
    if (msg.sender != _owner) {
      revert("not allowed");
    }
    _;
  }

  constructor() {
    _owner = msg.sender;
  }

  function getSecretMessage() external view onlyOwner returns (string memory) {
    return "Very secret message";
  }
}

After (on-chain auth)

import {SiweAuth} from  "@oasisprotocol/sapphire-contracts/contracts/auth/SiweAuth.sol"

contract MyContract is SiweAuth {
  address private _owner;

  modifier onlyOwner(bytes calldata bearer) {
    if (authMsgSender(bearer) != _owner) {
      revert("not allowed");
    }
    _;
  }

  constructor(string memory domain) SiweAuth(domain) {
    _owner = msg.sender;
  }

  function getSecretMessage(bytes calldata bearer) external view onlyOwner(bearer) returns (string memory) {
    return "Very secret message";
  }
}

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