services: define and use "scope"

[yarikoptic]

Came up in the context of

• Add neurosift external service for dandisets #2041
• Add webdav.dandiarchive.org to "services" for the main instance and redirect /download/ for zarrs to webdav  #1993 (comment)

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

• services: define and add categories for external services #2047