One version for all Firebase pods

[paulb777]

With the next major version, we plan to re-version all Firebase pods to 7.0.0 and then keep them in sync from then on.

This will align CocoaPods versioning with the Swift Package Manager, Carthage and zip distributions.

The implementation should replace all individual pod version handling and option processing with a call to the FirebaseCore pod to get the Firebase version.

Please add any comments or concerns to this thread.

[paulb777]

@ryanwilson Should we add a public API to return the current Firebase version?