Add method createImplicitOrderBy for ineqaulity filters

[HemangChothani]

Found this method in java and node which called in startAt, startAfter, endAt, endAfter functions but did't found similar in python, Also asked in this issue but at that time i misjudged the thing.

@crwilcox @tseaver Is that functionality needed for python?

[tseaver]

BaseQuery has methods start_at, start_after, end_at, end_before.

There is an explicit, documented requirement that all fields of cursors passed to those methods (or to the constructor) have matching entries in the query's order_by field. BaseQuery._normalize_cursor raises ValueError if that constratint is not met.

This issue would be a feature request to have _normalize_cursor synthesize the order_by bits, or else have the cursor-setting methods somehow implicitly set the order. I don't like the latter, as it requires guessing about ASCENDING / DESCENDING, and presumes that the order in which the methods are called should somehow dictate the sort order for the query.

[meredithslota]

Recategorizing as a feature request but it's not clear if this should actually be done or the relative priority.