HSEARCH-1724 Simplify access to ClassLoaderService

This is needed for the boot refactoring, the SearchFactory uncoupling from its SPI, and also avoids some map lookups during Query execution.

 - https://hibernate.atlassian.net/browse/HSEARCH-1724
This is spot-on one of the troublesome point I have with the whole bootstrap: exposing something like the ServiceManager to the *constructor* of core components like a `BridgeProvider` doesn't help to define a reasonable order in which all these things need to be started.

It would be much easier if we could narrow down the access to services as much as possible; in the specific case of the BridgeProvider, it obviously needs the `ClassLoaderService` but I don't see why it would need anything else...
I guess I could find some ideas, but none are "safe" in terms of other services needing to be initialized already.