Add provider definition for cheminfo namespace

[merkys]

A draft PR to introduce cheminfo namespace provider.

Cheminformatics namespace has seen a lot of attention in the 2025 OPTIMADE workshop. I believe it makes sense to move on with the registration of _cheminfo_ prefix with the required metadata. Furthermore, this is going to be an example case of namespace prefixes not tied to any data providers.

[rartino]

I'm happy to approve this, but is there a reason a namespace needs an index-metadb with a single base_url that is null, instead of just using null for base_url in providers.json?

[merkys]

I'm happy to approve this, but is there a reason a namespace needs an index-metadb with a single base_url that is null, instead of just using null for base_url in providers.json?

Right, this seems rather natural. I have added the index-metadb after talking to @ml-evs about the possibility to represent the namespace's properties in the [Providers dashboard])(https://www.optimade.org/providers-dashboard/). I think index-metadb is needed for it to work, but maybe this could be changed to not rely on index-metadb?

[rartino]

@merkys It seems potentially easier to patch the dashboard to recognize the difference between "database provider" and "namespace provider" already from providers.json, than to insist that all namespace providers we add need to set up the second-level indirection.