Make this beast into build/extensions helper?

[yarikoptic]

• it would provide
  • datalad_build_support module -- what now is _datalad_build_support/ within core and copied over iirc
  • commands
    • datalad extensions - list known/available extensions
    • datalad extension-template - populate extension template (might just clone existing template code, or use cookie cutter?)
    • datalad extension-regenerate - (re)generate all what it could from the extension description - setup.{cfg,py}, CI setups, etc.
• extensions.yaml would contain only a registry of extensions (urls) to their git repos
• each extension repository would contain actual details about itself in datalad_extension.yaml:
  • all the details necessary to produce out from the templates here