Accept partial Order object

[pouwerkerk]

This PR simply reduces the argument type requirements of getOrder(order: Order) from an object of type Order, to simply an object containing a URL (effectively Pick<Order, 'url'>). Only the Order URL is needed to retrieve an Order and re-create object, and that's how api uses getOrder anyway. Since api isn't exposed to consumers of acme-client, I propose this modest change.

Here's an example use-case:

const acme = require('acme-client');

const accountPrivateKey = '<PEM encoded private key>';

const client = new acme.Client({
    directoryUrl: acme.directory.letsencrypt.staging,
    accountKey: accountPrivateKey,
});

client.getOrder({ url: 'https://acme-provider.example.com/order/123' });

Unfortunately, TypeScript will warn the following:

Type '{ url: string; }' is missing the following properties from type 'Order': status, identifiers, authorizations, finalize

Looking at the type definition of the acme-client order, I see that it actually extends the rfc8555.Order definition:

export interface Order extends rfc8555.Order {
    url: string;
}

And creating a rfc8555.Order just to satisfy the function argument type requirements is onerous:

export interface Order {
    status: 'pending' | 'ready' | 'processing' | 'valid' | 'invalid';
    identifiers: Identifier[];
    authorizations: string[];
    finalize: string;
    expires?: string;
    notBefore?: string;
    notAfter?: string;
    error?: object;
    certificate?: string;
}

Thanks for your consideration!