Prior to this change the only RemoteConfig setting that was available is
the isDeveloperModeEnabled setting. On iOS this setting causes react
native firebase to call `initWithDeveloperModeEnabled` - which [is
deprecated](https://github.com/firebase/firebase-ios-sdk/blob/master/FirebaseRemoteConfig/Sources/Public/FIRRemoteConfig.h#L148-L149)
and appears to do nothing.
This adds support for the minimumFetchInterval setting, which is the
recommended way to control the fetch timeout now (on iOS at least)