Populate a "registry" of additional commands to be embedded within datalad

[yarikoptic]

ATM we have a hardcoded list of unknown commands to inform user which extension to install whenever some command is now "found": https://github.com/datalad/datalad/blob/master/datalad/interface/__init__.py#L94 . Obviously we would be missing newly added commands etc. I think we should make a cron job install all known extensions, harvest all additional commands which were brought by them into the entrypoints, and populate file like extensions_registry.py which then could simply be copied and used by the datalad core (a test within core or here could assure that the copy is up to date).