Today we allow plugins to add index store implementations yet we are not
doing this in our new way of managing plugins as pull versus push. That
is, today we still allow plugins to push index store providers via an on
index module call where they can turn around and add an index
store. Aside from being inconsistent with how we manage plugins today
where we would look to pull such implementations from plugins at node
creation time, it also means that we do not know at a top-level (for
example, in the indices service) which index stores are available. This
commit addresses this by adding a dedicated plugin type for index store
plugins, removing the index module hook for adding index stores, and by
aggregating these into the top-level of the indices service.
eb675a1c4d