Break function_score out into single- and multi-function types

[JoshMock]

Fixes #2109.

[l-trotta]

FunctionScoreQuery was recently updated with the @shortcut property which allowed us to accept every variant of the query, are we sure we need this?

[JoshMock]

I'm not entirely sure @shortcut_property covers all of this. As @swallez noted here, the issue is that all the properties of FunctionScoreContainer can be used on the same level as boost_mode, max_boost, etc. OR a functions property can be provided that includes one function object or an array of function objects.

[pquentin]

Thanks! LGTM.

Laura plans to take more time to review this.

[l-trotta]

we need to rename FunctionScoreContainer, the java generator (and maybe others?) removes the "Container" part of the name, and since FunctionScore already exists there's a duplicate error. Does FunctionScoreCommonContainer sounds okay?

Or renaming FunctionScore back to FunctionScoreQuery as is was before - feels weird being the only Query field that is not called SomethingQuery.