generate clone url from extsvc config with safe type assertions

Created by: arussellsaw

closes #20074 (closed)

looks like we had an instance of a nil repo.Metadata for a perforce repo 🤔

i've updated the type assertions to be safe, though i really wish you could do a double switch, eg:

switch a, b := foo.(type), bar.(type) {
 case Cool, Type: 
}