change MSafe accountkeys to multisgin form

[LeviHHH]

MSafe is a multi-sign wallet, but the form of the account we are now returning is similar to single-sign form, encoding multiple publickeys and threshold into a long hex string. This pr change the account form to:

interface Account {
    publicKey: string[];
    address: string;
    authKey: string;
    minKeysRequired: number;
}

And I also add MSafe link to README.

[JackyWYX]

More specifically, we are following the standard introduced here: https://aptos.dev/guides/wallet-standard/#dapp-api

interface PublicAccount {
    address: string;
    publicKey: string | string[];
    minKeysRequired?: number; // for multi-signer account
}