Add interface for stats and realtime analytics

[msuterski]

This adds an interface to the historical and realtime stats endpoints.

The tests do not verify any data because it would be different for any service or account. Just verify that the http requests are successful.

[sethvargo]

This looks really good - one question. Do you think realtimestats should be its own client? It feels a bit weird that these methods would be available on a regular client too. Or is that intended?

[msuterski]

The realtime stats are published from a different endpoint (api.fasty.com vs rt.fastly.com). I'm not sure how we could use the same client to talk to different hostnames? Is there a way?

I actually had it implemented as a separate type to be explicit which methods are available through which client type, but though it was a bit of an overkill. I can put it back.

[sethvargo]

Hey @msuterski

Yea - I know the RTS are different endpoints, which is why I think it makes sense for them to be different API clients.

[msuterski]

Ok, I'll push an update to this. Thanks!

[msuterski]

Updated to use a separate client type for the RTS API calls.

Let me know if it works for you, please.