Support creating signed urls for Cloud CDN

Originally filed in [nodejs-storage](https://github.com/googleapis/nodejs-storage/issues/1719) by @TriPSs
This is a request for a Node.js sample on [this page](https://cloud.google.com/cdn/docs/using-signed-urls#signing_urls)

Is your feature request related to a problem? Please describe.
We want to implement signed urls that go through the load balancer with Cloud CDN, this library does not yet expose a method that can be used to generate those urls.

Describe the solution you'd like
An method that will generate these urls for us.

Describe alternatives you've considered
Create the custom code inside the application itself.

Additional context
Would be awesome if this could be integrated inside the library as now everyone who needs this needs to figure it out themselves (As the Google Cloud docs does not have an example for node) and create their own solution.

Outstanding question:
```
if I understood it correctly you need to create a custom "URL signing key" for the load balancer and use that one when creating the signed url so Cloud CDN / Load Balancer can accept it. Or is this incorrect?
```