Allow for customising rest clients

[Mattchewone]

Supercedes #1776

Allows for customising the rest client:

const feathers = require('@feathersjs/feathers');
const rest = require('@feathersjs/rest-client');
const { FetchClientService } = ​rest;

const app = feathers();
​
class MyFetchClient extends FetchClientService {
  getQuery () {
    return {};
  }
}
​
// Connect to a different URL
const restClient = rest('http://feathers-api.com');
​
// Configure an AJAX library (see below) with that client 
app.configure(restClient.fetch(window.fetch, MyFetchClient));
​
// Connect to the `http://feathers-api.com/messages` service
const messages = app.service('messages');

[DaddyWarbucks]

I believe this was already completed via #1594

But, this PR does actually have a few extra thins like tests, etc. So perhaps it is better to merge this one as well.

EDIT
Disregard my previous comments. I see this PR actually "finishes" the other PR by making that PR's change more accessible.

[daffl]

Yup, I think they work together well. I think this also relates to #1102 to make it easier to initialize services from multiple REST endpoints.

[daffl]

Nevermind, I realized that this has been possible properly for years already 😄 This PR has been released in v4.5.0, thank you again!