FR: Add support for Cloudflare's D1 in database URIs

[pataquets]

D1 is Cloudflare’s native serverless database.
Info: https://developers.cloudflare.com/d1/

There is an API and at least a couple wrapper libs:
https://developers.cloudflare.com/d1/examples/query-d1-from-python-workers/
https://github.com/G4brym/django-d1
https://github.com/Suleman-Elahi/D1py

I guess there's no hurry, just exploring. If it would be an acceptable PR, general advice is appreciated. Use d1:// schema? If too out-of-scope, is there any extensibility feature to plug in a new scheme?
I guess this could be tagged as a "good first issue", since looking at the code, seems to me that it's just the db_url parsing function.