Add missing implementations for iOS/Android on performance

[blackwiz4rd]

Hello,
I'm willing to add the missing implementations for Firebase Performance SDK for iOS/Android.
So I will try to make a PR as soon as possible for this feature.
I hope it will be helpful.
Regards!

[blackwiz4rd]

I'm almost done with the implementation.
Missing tasks:

• iOS implementation for HttpMetric
• Run tests on Android

If you guys want to see my live implementation checkout https://www.youtube.com/watch?v=kR--EWdbHng, it might inspire you to contribute to this project as well :)