feat(behavior): add support for prefetch action

[palaparthi]

Add support for new action called prefetch, this will fetch an URL in the background.
Offline mode(built internally at Instawork) will take advantage of this by prefetching the necessary screens and saving them in the cache.

[hgray-instawork]

Not in scope for this but I wonder if the behaviors should receive the entire onUpdateCallbacks object?

[palaparthi]

Not in scope for this but I wonder if the behaviors should receive the entire onUpdateCallbacks object?

I thought about it too but decided otherwise since that needed a refactor but I agree with you.

@flochtililoch suggested that this is probably better suited as a custom behavior since this is not documented/available for public use. I will park the PR for now and come back to this later once I have offline write stuff working.