Core: deferred rendering

[dgirardi]

Type of change

• Feature

Description of change

This introduces the ability to defer rendering of bids until their billing is explicitly triggered through pbjs.triggerBilling. This is to allow "pre-rendering" (in formats like interstitial) of bids that fire billing pixels when rendered.

• update the pbjs.triggerBilling API to accept ad units in addition to specific bids (e.g. triggerBilling({adUnitCode: '...'}) will trigger whichever bids won for that ad unit)
• bids on ad units that have deferBilling: true, but whose bid adapter does not define a billing method (onBidBillable), are marked for deferred rendering.
• when either a remote creative or a call to pbjs.renderAd attempts to render such a bid, it is marked as winning (BID_WON is emitted, it's added to pbjs.getAllWinningBids(), etc), but it is not actually rendered.
• the actual rendering only happens when pbjs.triggerBilling is called.

Other information

Closes #10594

[github-actions]

Whoa there, partner! 🌵🤠 We wrangled some duplicated code in your PR:

• modules/bidViewability.js has 10 duplicated lines with modules/dfpAdpod.js

Reducing code duplication by importing common functions from a library not only makes our code cleaner but also easier to maintain. Please move the common code from both files into a library and import it in each. Keep up the great work! 🚀