Make `ServiceKeys`, `ServiceTypes`, `ServiceLifetimes` internal to prevent external mutation

[AGiorgetti]

Description

ServiceKeys, ServiceTypes, ServiceLifetimes, and ServiceKeys<T> in ServiceProviderFactory.cs are declared as public classes that inherit from HashSet<>. This exposes mutable collection APIs (.Add(), .Remove(), .Clear()) to all consumers of the library.

public class ServiceKeys<T> : HashSet<object> { ... }
public class ServiceKeys : HashSet<object>;
public class ServiceTypes : HashSet<Type>;
public class ServiceLifetimes { ... }  // has public Add() method

Impact

Library consumers can mutate these tracking collections after the service provider is built, potentially corrupting the IsServiceRegistered / IsKeyedServiceRegistered / lifetime-check infrastructure.

Suggested Fix

These types are only used internally by ServiceProviderExtensions. Consider:

1. Making them internal (they're registered as singletons and resolved by type, so internal visibility is sufficient)
2. Or wrapping them in read-only interfaces for the public API

Note: ServiceKeys<T> needs to remain resolvable by the generic GetAllServices method, but that can work with internal visibility since both are in the same assembly.