[4.x] Provide searchable entries and terms lazily

[ryanmitchell]

Now that the query builder supports lazy, the searchable provider provide() method can make use of that to return documents in a more memory efficient manner.

The PR updates the interface to allow this method to return a LazyCollection and updates both entries and terms to do that. It seems this is not a breaking change, as other providers still return a standard collection and I encountered no errors.

I notice that assets and users are not using a query builder (they are just returning a filtered all()), so there is definitely scope to improve things further by updating those providers to do that.

This should close #8854