feat(model-list): be consistent, skip known files from listing (#2760)

fix(model-list): be consistent, skip known files from listing

This changeset does two things:

- Removes the dependency of listing models from the OpenAI schema.
- Tries to reduce confusion between ListModels() in model loader and in
  the service - now there is only one ListModels which is in services
and does not depend anymore on the OpenAI schema
- The OpenAI-schema functions were moved nearby the OpenAI specific
  endpoints that needs the schema
- Drops the ListModel Service structure as there was no real need for
  it.

Signed-off-by: Ettore Di Giacinto <mudler@localai.io>