services: define and use "scope"

Came up in the context of
- https://github.com/dandi/dandi-archive/pull/2041
- https://github.com/dandi/dandi-archive/issues/1993#issuecomment-2414421776

we could add services not only for assets but also
- `dataset` (generic, non-DANDI specific term so could later be reused by others)
- `directory` ("folder" is forbidden in BIDS)
- `file` (generic, non-DANDI specific)

so then corresponding selection of services could be picked up.

I think it is ok to have `scope: str` (not a `list[str]`) since for every scope there would be different URL and potentially descriptive "categories", as per
- #2047

Read pull request #2041, issue #1993 and its linked comment, and issue #2047 first. Resolve how dataset, directory, file, and asset scopes map to service selection, URLs, and descriptive categories, then define the scope behavior and its completion criteria.

Written by the indexing model from the issue text.