Definition namespaces and providers

[ml-evs]

This PR begins the work on #328, adding prefixes that can be shared across providers. I will try to follow this rough plan with my work:

• Define "definition provider" [working title]
• Replace relevant mentions of "database-provider-specific" with "namespace-specific"
• Potentially extend the provider/link definition to add a specialism for this kind of provider (decided against it)
• Describe mechanism for becoming a definition provider
• Provide example cases

Then outside of the specification we need to:

• Set up an example definition provider, as a separate repository, that can be modified for future domain-specific extensions

Closes #328

[ml-evs]

I think this is ready for some initial feedback. I've also made the start of a namespace repo at https://github.com/ml-evs/optimade-stability-namespace (currently forked to https://github.com/Materials-Consortia/optimade-stability-namespace) where I am trying to use @rartino's processing tools from #445 to generate the website/schemas (currently with no luck).

[rartino]

@ml-evs I've gone through this, and aside the minor formatting issue @merkys remarked on (and I've provided a diff for), this looks very good to me and I don't quite see why we cannot already merge it.

The new optimade-property-tools repo with its example directory may make it a bit easier to run a namespace provider test.

[ml-evs]

Thanks @rartino, happy to merge this before rc.2 so we can test out the implications.

[rartino]

This looks good to me. We are already trying this out in other repos.

[merkys]

I made some minor suggestions to put definition-provider prefixes to the same level as database-provider prefixes. AFAIK they should be roughly equivalent from now.

[merkys]

Good to go, thanks!

[rartino]

Re-confirming my positive approval.

[ml-evs]

Ok, now that the one linting issue is fixed (with the checker introduced since your last reviews...) I think this is the final time I need to bother you all with review requests (on this PR...)