Support for mirroring/extending this providers list

[ml-evs]

In #71 we are seeing the problem with having a single hosted list for the providers. I suggest that we convince 1-2 of us to host an alternative providers list (not necessarily mirrors, but at least 1 should be). These could be added directly to providers.json here with "link_type": "providers" (which would then allow us to link together federations of OPTIMADE providers...). Strict mirrors of this list, e.g., someone who is commmited to keeping an up-to-date git submodule of this repository hosted online somewhere outside of optimade.org, should then also be listed elsewhere in this repository.

I am happy to do the latter (though probably won't have the time for a few weeks) if we think this is a good idea.

[rartino]

Not that I have anything against this. But if we are anyway working on this side of the infrastructure - wasn't the idea that every single database would mirror the providers list via the links endpoint? The hope would in the end be that we'd end up more with a distributed network of providers all linking each other.