config: Add bytes JSON to storagepb.Group, parse YAML config

* Allow bootcfg groups metadata to be defined via YAML
* Convert YAML metadata to JSON bytes field in a protobuf message
* Strip metadata except strings, string slices, and string-keyed maps
with nesting

cmd/bootcfg-rpc/main.go

@@ -74,7 +74,7 @@ func main() {
 	// storage
 	store := storage.NewFileStore(&storage.Config{
 		Dir:    flags.dataPath,
-		Groups: cfg.PBGroups(),
+		Groups: cfg.Groups,
 	})
 
 	// gRPC Server

cmd/bootcfg/main.go

@@ -103,7 +103,7 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
-	store.BootstrapGroups(cfg.Groups)
+	store.BootstrapGroups(cfg.YAMLGroups)
 
 	// HTTP server
 	config := &api.Config{