[Draft] custom paymaster handler & zyfi integration

[0xSulpiride]

Description

Added paymaster plugin support that enables developers to create external paymaster plugins that integrate with SDK easily.

Key changes:

• New CustomPaymasterHandler type and integration in transaction flow
• Example implementation with ZyFi's paymaster API

wip: add paymaster handler to passkey clients

Additional context

With these changes users can start using external paymasters easily with just 2-3 lines of code

import { zksyncSsoConnector, callPolicy } from "zksync-sso/connector";
import { zksyncSepoliaTestnet } from "viem/chains";
import { createConfig, connect } from "@wagmi/core";
import { erc20Abi } from "viem";
import { createZyfiPaymaster } from "zksync-sso/paymaster";

const zksyncConnectorWithSession = zksyncSsoConnector({
  authServerUrl: "http://localhost:3002/confirm",
  session: {
    feeLimit: parseEther("0.1"),
    transfers: [
      {
        to: testTransferTarget,
        valueLimit: parseEther("1"),
      },
    ],
  },
  paymasterHandler: createZyfiPaymaster({
    apiKey: 'xxxxx-xxxxx-xxxx',
    sponsorshipRatio: 100
  }),
});
const zksyncConnector = zksyncSsoConnector({
  authServerUrl: "http://localhost:3002/confirm",
  paymasterHandler: createZyfiPaymaster({ apiKey: 'xxxxx-xxxxx-xxxx' }),
});
const wagmiConfig = createConfig({
  chains: [chain],
  connectors: [zksyncConnector],
  transports: {
    [chain.id]: http(),
  },
});

How it works:

customPaymasterHandler acts as a middleware which is triggered just before signing a transaction.
Simplest paymaster handler will just add static paymaster and paymasterInput to a transaction.

const zksyncConnector = zksyncSsoConnector({
  authServerUrl: "http://localhost:3002/confirm",
  paymasterHandler: async () => ({
    paymaster: PaymasterContract.address,
    paymasterInput: getGeneralPaymasterInput({ innerInput: "0x" }),
  }),
});

The more complex one, like ZyFi's, will send the transaction to some backend service and can fetch not only paymasterInput, but also gas fees since some paymasters may rely on exact values of these fields to account for ERC20 tokens.