Implemented FCM Webpush API

[hiranya911]

• Implemented WebpushConfig and WebpushNotification types
• Removed an unused extension method (IEnumerable.Copy())
• Made some internal properties private for better encapsulation

[stewartmiles]

I'm wondering why you have a set of nullable properties for types like this? Why not use default values instead? To me it seems like a pain to deal with the nullability of e.g bool properties rather than just reading the default value.

[hiranya911]

This is to make sure those parameters get dropped from the JSON output. For most parameters, we want to avoid sending a default value to the backend (0 for ints, false for bools etc) when they are not set by the user.

[stewartmiles]

It does worry me a little that we're letting optimizations leak into the API surface. I guess if that's the way you do it across the board it's ok to me.

[bklimt]

LGTM. I'll leave the review to Stewart.