Attribute/IdPAttribute

[Tom Zeller]

> [Snip due to time]

How about a convention that interfaces in the IdP are named of the
form "IdP<Type>", meaning add IdPAttribute and IdPRequestedAttribute
as interfaces, as well as IdPDataConnector and IdPAttributeDefinition,
and I guess IdPAttributeResolver and IdPAttributeFilter if we think
so.

Maybe these interfaces will ease upgrades, maybe we provide a default
implementation to map selected interfaces from v2 to v3.

We probably should then rename classes like BaseAttributeDefinition to
DefaultAttributeDefinition implementing IdPAttributeDefinition.

Being lazy, this seems like a reasonably small amount of work to
address several issues that have been raised.

Happy to discuss, being brief due to time.