add server(s) to openapi.json

The OpenAPI definition supports providing a list of API servers inside the openapi.json.

Adding a list of servers there then allows the Swagger documentation to select the server and actually start testing requests.

See here for a super quick test - it does not yet fully work because the optimade API needs to set the Access-Control-Allow-Origin: * header (which the Materials Cloud optimade servers currently don't do, but should! @CasperWA ).

I have a couple of small suggestions:

• figure out how to add the list of servers programmatically (I just edited the openapi.json). this should be easy
• we can add servers from optimade providers to this list but, to start with, I would add only servers that set the 'Access-Control-Allow-Origin' header. While it is currently not demanded in the spec, this can be a positive "nudge" for providers to do this
• top of the list should be a neutral "sample" server. We could use the heroku instance for this but it spins down after 30 minutes of inactivity. @blokhin if zeit.co has better conditions, do you think it is better to move from heroku there?
• My playing around with this showed a flaw in the current openapi.json - all paths start with optimade/ and so the openapi.json enforces the optimade/ prefix. This is not mandated by the spec and the optimade/ prefix should be removed.
  I would probably even remove the /v0 prefix from the paths in the openapi.json, since - although mandated - it can be set by the provider. It makes more sense to specify a base URL https://dev-aiida-dev.materialscloud.org/curated-cofs/optimade/v0 than to specify a base URL https://dev-aiida-dev.materialscloud.org/curated-cofs/optimade).

If we put these few little points together, we should be rewarded with a fully functional online documentation that is integrated with a browser client for querying the databases provided in the list.
I think this would be very useful.

Mentioning also @ml-evs for info

Start with openapi/openapi.json and the linked Swagger UI test, then trace how the definition is generated so the server list is not hand-edited. Review the requested server selection and removal of the optimade/ and possibly /v0 path prefixes. Done means the generated documentation exposes usable servers and the paths work against the selected base URL.