allow custom global key for Koala

[mshwery]

We've found some instances where customer code encounters conflicts with window.ko and we're modifying the install snippet to respect their custom globalKoalaKey.

Testing

Tested this version of the init script manually. Koala can load with a custom namespace when window.globalKoalaKey is set.

• Added unit tests for new functionality
• Tested end-to-end using the local server
• [If destination is already live] Tested for backward compatibility of destination. Note: New required fields are a breaking change.
• [Segmenters] Tested in the staging environment
• [Segmenters] [If applicable for this change] Tested for regression with Hadron.