Introduce top level service locator.

Introduce top level service locator.

The service locator, MediaWikiServices, is intended to facilitate
"manual" dependency injection in static entry points.

See also the Dependency Injection RFC T384 and Service Locator
RFC T124792 for details.
The following key points were implemented according the
discussion surrounding these RFCs:

* a configurable DI container that allows extensions to add and replace services.
* no auto-wiring, since it's prone to add confusion in large and complex applications.
* no 3rd party framework, since they typically do too much.

The following services in MediaWiki core are made accessible via the service locator
mechanism to showcase the bootstrapping mechanism:

* ConfigFactory and MainConfig
* SiteLookup and SiteStore

However, the implementation of these services was not yet converted to using proper DI
throughout the code.

Bug: T124792
Change-Id: I3c25c0ac17300d3dd13e1cf5100558a605eee15f