panic for unix:/// addresses

```
$ grpcdebug unix:///home/howardjohn/kube/local/proxy/XDS channelz servers
panic: Address type not supported for uds_address:

goroutine 1 [running]:
github.com/grpc-ecosystem/grpcdebug/cmd.prettyAddress(0xc000113440, 0xc0006813e0, 0xc000595cf8)
/home/howardjohn/go/pkg/mod/github.com/grpc-ecosystem/grpcdebug@v1.0.1/cmd/channelz.go:40 +0x145
github.com/grpc-ecosystem/grpcdebug/cmd.channelzServersCommandRunWithError(0x16ceba0, 0x170ca70, 0x0, 0x0, 0x0, 0x0)
/home/howardjohn/go/pkg/mod/github.com/grpc-ecosystem/grpcdebug@v1.0.1/cmd/channelz.go:320 +0x170
github.com/spf13/cobra.(*Command).execute(0x16ceba0, 0x170ca70, 0x0, 0x0, 0x16ceba0, 0x170ca70)
/home/howardjohn/go/pkg/mod/github.com/spf13/cobra@v1.1.1/command.go:850 +0x472
github.com/spf13/cobra.(*Command).ExecuteC(0x16cf380, 0xc000000180, 0x200000003, 0xc000000180)
/home/howardjohn/go/pkg/mod/github.com/spf13/cobra@v1.1.1/command.go:958 +0x375
github.com/spf13/cobra.(*Command).Execute(...)
/home/howardjohn/go/pkg/mod/github.com/spf13/cobra@v1.1.1/command.go:895
github.com/grpc-ecosystem/grpcdebug/cmd.Execute()
/home/howardjohn/go/pkg/mod/github.com/grpc-ecosystem/grpcdebug@v1.0.1/cmd/root.go:107 +0xc5
main.main()
/home/howardjohn/go/pkg/mod/github.com/grpc-ecosystem/grpcdebug@v1.0.1/main.go:160 +0x25
```

other commands seem to work with UDS, so this seems unexpected, especially since its just on trying to print the address