FR: Support for Mac OS Safari Extension

Thank you very much for adding Mac OS support for firebase SDK.

I have an Mac OS Safari Extension from which I'd like to call `FirebaseApp.configure()` with its own bundle identifier.
In MacOS Safari extension, we define `SafariWebExtensionHandler.swift` and I call `FirebaseApp.configure()` from its `beginRequest` function.

However, I get an exception, as `FirebaseApp.configure()` is called many times, as every time `popup.html` is loaded from the extension icon in the Safari Browser and calls native, separate instances of `SafariWebExtensionHandler` is created. I suppose this is similar to iOS app where multiple `ActionViewController` instances are created.

However, for the case of iOS, `[GULAppEnvironmentUtil isAppExtension]` returns `true` and exception is not thrown while it returns `false` in MacOS Safari Extension. Would it be possible to support the same feature for MacOS Safari Extension? Thank you.