Allow faker options (including seed) to be passed via request headers

[awdyson]

I'd love to use Prism as part of an integration test approach.
Imagine a Puppeteer test that could give shaped, repeatable responses...

page.on('request', (request) => {
    const seed = '123';

    const headers = request.headers();
    switch (request.url) {
        case 'endpoint_1':
            headers['x-json-schema-faker'] = { seed, 'min-items': 10 };
        case 'endpoint_2':
            headers['x-json-schema-faker'] = { seed, 'min-items': 1, alwaysFakeOptionals: true };
    }

    request.continue({ headers });
});