ContextSuggestField indexes context information along with
suggestions, which can be used to filter and/or boost suggestions using
It would be useful to make ContextSuggestField extensible such that subclasses
can inject context values at index-time, without having to specify the
contexts in its ctor.
ContextQuery can be made extensible by allowing sub-classes to override
how context automaton is created from provided query contexts.
Currently, ContextQuery uses a context value of "*" to consider all context values,
It makes sense to have a dedicated addAllContexts() instead.