Do not validate extension endpoints

[ml-evs]

With the registration of MPDS, we now have our first (?) provider that serves custom endpoints through OPTIMADE, e.g. https://api.mpds.io/v1/extensions/phase_diagrams. All bets are off as to what these endpoints should serve, so we should not check them with the validator.

[ml-evs]

Potential open question: is the below correct, or should the entry types by format be provided without the extensions/ prefix? I think this is ambiguous in the specification...

 "data": {
    "type": "info",
    "attributes": {
      "api_version": "1.0.0",
      "available_api_versions": [
        {
          "url": "https://api.mpds.io/v1",
          "version": "1.0.0"
        }
      ],
      "available_endpoints": [
        "info",
        "structures",
        "extensions/properties",
        "extensions/phase_diagrams",
        "extensions/phases"
      ],
      "formats": [
        "json"
      ],
      "entry_types_by_format": {
        "json": [
          "structures",
          "extensions/properties",
          "extensions/phase_diagrams",
          "extensions/phases"
        ]
      }
    }
  }

[CasperWA]

I'd say the extensions/ should be part of the entry_types_by_format, but maybe you're right in asking this, as the resource entry types definitely do not include the extensions/ part.
Indeed, the more I think about it, it doesn't make sense, I guess. It only makes sense for the available_endpoints.