Deserialization regressions: cannot resolve child databases in client

[ml-evs]

In "v1", the client can no longer resolve child databases, since when deserializing a LinksResponse, the LinkType enum is not present in the final model so a filter that checks link["attributes"]["link_type"] == LinkType.CHILD does not trigger.

I'm slightly worried that this opens us up to many more occurrences of this, so I would say this is a blocking issue. Our tests did not run into it as we do test the client on "live" provider data (maybe we should -- the test could just be whether there are ANY accessible OPTIMADE APIs out there, if this test fails for non-code reasons then we can probably roll up this package anyway 🙃)