[nodejs] Add support for apiEndpoint override

[JustinBeckwith]

Today, we allow developers to override the service endpoint using the servicePath variable in the service client constructor:

const client = new ImageAnnotatorClient({
  servicePath: 'vision.not-googleapis.com'
});

In go/extensible-options-in-nodejs, we discuss changing this to apiEndpoint, as it's consistent with existing hand written libs, and feel that it's a little more obvious:

const client = new ImageAnnotatorClient({
  apiEndpoint: 'vision.not-googleapis.com'
});

Ideally, this wouldn't be a breaking change. In the generated code, it should look something like this:

const servicePath = 
  opts.servicePath || 
  opts.apiEndpoint || 
  this.constructor.servicePath;

opts = Object.assign({
  clientConfig: {},
  port: this.constructor.port
}, opts);

If we're feeling 🌶 , we could also add the apiEndpoint getter to match the contents of servicePath.