Introduce the Restbase Virtual REST Service class

Introduce the Restbase Virtual REST Service class

Restbase, the REST content API service, is to be queried instead of
Parsoid by current Parsoid users (most importantly VE). This patch
introduces the Restbase virtual REST service class and transparently
maps Parsoid calls into Restbase ones if parsoidCompat is set when
creating the service object.

Additionally, $wgVirtualRestConfig is introduced in DefaultSettings.php. This
is a first step towards global service configuration and management.

Bug: T89066
Change-Id: I4d4043e5052327bbd789331f1c05b607c45fe7cb