Move GetManyExtensions to IElasticClient interface

[rikkit]

Would you consider a PR to move the extension methods from GetManyExtensions to the client interface? As it stands these methods are impossible mock out in unit testing using Rhino (and other mocking frameworks that can't deal with static methods).

[Mpdreamz]

I mostly want to keep IElasticClient free of any of the "convenience" methods that do not really translate to an Elasticsearch API endpoint. I'm dead set against though, @gmarz @russcam you guys have a strong opinion on this?

[russcam]

I think it's prudent to keep any kind of convenience methods separate from the client in order to keep methods aligned with the Elasticsearch API.

That said, we do have ReIndex<T> on the client directly in master (though a ReIndex API is being proposed) 😄

[gmarz]

Agreed for the same reasons above.

[Mpdreamz]

I meant to say I'm not dead set against :D We might re-evaluate come 3.0.