Relocates client.ParseLabelSpec to lib/utils/parse. This results in
a net 5MB reduction in the weight of lib/config. This may not
have a correlation in binary size since lib/client is heavily consumed
by other packages.
```bash
$ goda cut './lib/config:all' | rg '^github.com/gravitational/teleport/lib/client\b|^ID'
github.com/gravitational/teleport/lib/client InDegree=1 Cut.PackageCount=75 Cut.AllFiles.Size=5.3MB Cut.Go.Lines=129512
```
rosstimothy
58126ad723