Enable DAO.new to register unmigrated schemes

The use case: One wants to create a new DAO and simulteneously register a scheme whose constructor requires an avatar address. Currently the only way to register such a scheme is to create the DAO and then propose to add the scheme via `SchemeRegistrar`.

So this ticket proposes that `DAO.new` support the provision of a callback that the caller will implement to instantiate a scheme once the avatar address is known. `DAO.new` will supply the avatar address as a parameter to the callback. The callback will return the new scheme's address and `DAO.new` will proceed to register it with the new avatar's controller.

Start by locating the DAO.new entry point and the SchemeRegistrar integration, then trace how the avatar address and controller are handled during DAO creation. Implement the callback flow described in the issue so a scheme can be instantiated after the avatar is known and registered before DAO.new returns; add or update tests covering callback registration and the existing path.

Written by the indexing model from the issue text.