API: harmonize plurality of endpoints

- `assets`
- `blobs`
- `dandisets`
- `uploads`

but then
- `zarr`

IMHO should be `zarrs` if simply for consistency unless someone shows `dict` definition that plural of `zarr` is `zarr` ;)

PS FWIW I think we should have gone for singular for all of them, but too late for that

Start by locating the API route definitions for assets, blobs, dandisets, uploads, and zarr. Check how their plural names are declared and update the inconsistent endpoint naming, then run the relevant API tests if the project provides them; done means the endpoint names follow one consistent convention.

Written by the indexing model from the issue text.