Cleanup nested, has_child & has_parent query builders for inner hits construction

[martijnvg]

• Inner hits are now only provided and prepared in the constructors of the nested, has_child and has_parent queries. This will make fixing has_child and inner_hits for grandchild hit doesn't work #11118 easier and then allow us to drop the top level inner hit syntax.
• Also made score_mode a required constructor parameter. (fromXContent(...) method maintain their defaults)
• Moved has_child's min_child/max_children validation from doToQuery(...) to a setter. (so we can fail sooner on the coordinating node)

[colings86]

QueryBuilders should always set the defaults since they can be constructed without ever passing through the parser (Java API)

[martijnvg]

• I think score_mode should be a constructor parameter, as it is important to always think about that.
• I'll move inner hits to a setter.
• I'll bring back the defaults for min and max kids.

[colings86]

Apart from my comments about query builder default values this looks pretty good. Am I right in saying that these QueryBuilders are effectively immutable now? (i.e. they have no setters?). If so, I think #17748 will change that as ignore_unmapped is not mandatory. I am also wondering why you needed to move score and InnerHitsBuilder to the constructor?Is there a reason they cannot be set after construction? I only ask as it is nice if only the mandatory parameters are specified in the constructor so the user doesn't have to worry about what the sensible defaults are.

[martijnvg]

@colings86 Only score_mode will additionally be made a mandatory parameter. The other params remain optional.

[colings86]

LGTM