db: implement fmt.Stringer on BackingType

This will be used within Cockroach to implement a custom JSON serialization of
the set of sstables returned by the sstable_metrics builtin.

db.go

@@ -2098,8 +2098,21 @@ const (
 	// or lifecycle management. An example of an external file is a file restored
 	// from a backup.
 	BackingTypeExternal
+	backingTypeCount
 )
 
+var backingTypeToString = [backingTypeCount]string{
+	BackingTypeLocal:         "local",
+	BackingTypeShared:        "shared",
+	BackingTypeSharedForeign: "shared-foreign",
+	BackingTypeExternal:      "external",
+}
+
+// String implements fmt.Stringer.
+func (b BackingType) String() string {
+	return backingTypeToString[b]
+}
+
 // SSTableInfo export manifest.TableInfo with sstable.Properties alongside
 // other file backing info.
 type SSTableInfo struct {