Provide cfg_duct procedure for DataLad

which would create and configure .duct/ subdataset, potentially reusing some global duct config or datalad config on what cfg_ to use for it (e.g. cfg_text2git), and also potentially adding some other settings like submodule extra properties for .gitmodules.

Original use case where I thought to use it is annextube so we could initiate consistently those '.duct/'s without hardcoding "duct specifics" into annextube and rather ask to provide procedure name!