Define/maintain centralized registry of instances

Inspired by
- https://github.com/dandi/dandi-cli/pull/1519

I wonder if to avoid even the need to modify dandi-cli to add a new instance, we could/should define a centralized registry of instances, e.g. in this repo, and make `dandi-cli` fetch up-to-date version once in a while?

we could define a versioned schema for the records to verify, and if anything goes wrong -- use "hardcoded" in the code copy.

WDYT @aaronkanzer @kabilar @jwodder

In the longer run we might also allow to register additional instances locally, hence also filed
- https://github.com/dandi/dandi-cli/issues/1521