Allow for custom trackerName for Google Analytics

[ianks]

What does this PR do?

Allows for users to set a custom tracker name for Google Analytics (other than segmentGATracker).

Are there breaking changes in this PR?

No.

Any background context you want to provide?

We would like to use a different name, and this PR allows us to do so.

Is there parity with the server-side/android/iOS integration components (if applicable)?

n/a

Does this require a new integration setting? If so, please explain how the new setting works

Yes. Users can now set trackerName: "MyTrackerName" which will name the tracker in GA.

Links to helpful docs and other external resources

https://developers.google.com/analytics/devguides/collection/analyticsjs/creating-trackers#working_with_multiple_trackers