Add a central ID lookup service

Add a central ID lookup service

Anything that wants to be "central" right now has to depend on
CentralAuth, and then either can't work without CentralAuth or has to
branch all over the place based on whether CentralAuth is present. Most
of the time all it really needs is a mapping from local users to central
user IDs and back or the ability to query whether the local user is
attached on some other wiki, so let's make an interface for that in
core.

See I52aa0460 for an example implementation (CentralAuth), and Ibd192e29
for an example use (OAuth).

Bug: T111302
Change-Id: I49568358ec35fdfd0b9e53e441adabded5c7b80f