Adds metamask example

[guillermodoghel]

Adds an example on how implement embedded checkout using a direct metamask connection

signAndSendTransaction: async (transaction: Transaction) => {
                                if (!window.ethereum) {
                                    throw new Error("No ethereum provider");
                                }
                                return await window.ethereum
                                    .request({
                                        method: "eth_sendTransaction",
                                        params: [{
                                            from: account,
                                            to: transaction.to,
                                            value: transaction.value._hex,
                                            data: transaction.data,
                                        }],
                                    }) as string;
                            },