Add Extended OSV info

[celek ibm]

I was looking at this 

https://api.osv.dev/v1/vulns/GHSA-25pr-6pr6-68v7

And wanted to retrieve the Severity - not sure if the osv.go is the right place to code something like this 

```

default: adb := a.Database // check if Severity exists if so , use it var X map[string]json.RawMessage if err := json.Unmarshal([]byte(adb), &X); err != nil { continue } var str string if err := json.Unmarshal(X["Severity"], &str); err != nil { continue } else { proto.Severity = str proto.NormalizedSeverity, err = fromString(str) } continue }

```

[Hank Donnay]

Yes, I think that's the right place to do something like that.

--
hank