Expose setUserProperties to set multiple user properties at once

[dlee]

Is your feature request related to a problem? Please describe.
There's currently no way to set user properties at once.

Describe the solution you'd like
I'd like a FirebaseAnalytics.setUserProperties({ [name]: [value], ... }) method exposed.

Describe alternatives you've considered
The only alternative is to iterate through all the properties and set them one at a time using FirebaseAnalytics.setUserProperty. However, this seems inefficient given that the underlying library seems to support setUserProperties.