Why:
Temporary accounts introduced as part of IP Masking are supposed
to expire 1 year after their registration. Accounts can have
multiple registration timestamps when in a wiki-farm setup,
depending on which wiki we consult.
To implement the account expiration logic, we need to know the global
(first) registration timestamp.
Similar to CentralIdLookup, a concept of a registration date
provider is introduced. Unlike IDs, users can have multiple
kinds of registration dates (Wikimedia recognizes local and global,
but third parties can have different needs). For that reason,
any number of registration providers can be registered at any
given time; caller determines which one is requested. The default
is `local`, which is the only provider that is guaranteed to exist.
What:
* Add UserRegistrationLookup
* Add UserRegistrationProviders config variable
Bug: T344694
Change-Id: If9fa12a392064dd504590a861a175e3604a34fab
