Core & PBS Adapter: support eventtrackers, and normalize burl / ext.prebid.events.win into it

[dgirardi]

Type of change

• Feature

Description of change

The PBS Adapter had special handling for two tracking URLs:

• bid.burl, which was fired when a bid is deemed billable;
• bid.ext.prebid.events.win, fired when a bid is rendered.

This allows bid responses (from any adapter) to provide an eventtrackers array as described in #12216:

• ortbConverter now moves imp.ext.eventtrackers into bidResponse.eventtrackers;
• the PBS extensions for ortbConverter translate burl into an impression pixel and events.win into a "win" pixel (custom event 500), appending them to eventtrackers;
• core fires win pixels on render, and impression pixels on billing;
• markWinningBidAsUsed's analytics flag is renamed to events, and triggers both when appropriate.

Other information

Closes #12698 (comment)
Closes #12216

[github-actions]

Tread carefully! This PR adds 1 linter warning (possibly disabled through directives):

• src/eventTrackers.js (+1 warning)