Add ARA source registration to VAST demo

[jolynyao]

Is your feature request related to a problem? Please describe.

I'd expect it to be very common to want to register sources for the Attribution Reporting API for instream video ads.
There are guidelines available here but it would be really useful to add to the existing demo.

Describe the solution you'd like

Add ARA source registration to the VAST / PA demo

[samdutton]

Thanks for the suggestion!

@kevinkiklee FYI.