AthToolSupport: support Gaudi interface base classes

Add support for the usage of the Gaudi extends formalism for interface declarations in dual-use code:

• Introduce the new AsgTools/Interfaces.h header that contains the standalone versions of these classes. Also move the DeclareInterfaceID here.
• This removes the need to use the explicit declareServiceInterface calls.

The first commit is the substantial change. The remaining commits are just migrating clients. This change will obviously break any code out there that is still using declareServiceInterface.

cc @krumnack