I2I: Create a `pbjs_global` that would store pbjs globals on page

## Type of issue
Feature

## Description
Since renaming of the pbjs global is a supported feature, it would be nice if we had a reference on a page to what the global entry was. 
Proposal: Store each pbjs global name in a `pbjs_global` array. 
ex: 
```
//create a pbjs global pointer
window.pbjs_global =  window.pbjs_global || [];
window.pbjs_global.push('$$PREBID_GLOBAL$$');
```