fs: implement optional Metadata interface for Objects #111

This implements integration tests for the feature also.

fs/metadata.go

@@ -0,0 +1,29 @@
+package fs
+
+import "context"
+
+// Metadata represents Object metadata in a standardised form
+//
+// See docs/content/metadata.md for the interpretation of the keys
+type Metadata map[string]string
+
+// Set k to v on m
+//
+// If m is nil, then it will get made
+func (m *Metadata) Set(k, v string) {
+	if *m == nil {
+		*m = make(Metadata, 1)
+	}
+	(*m)[k] = v
+}
+
+// GetMetadata from an ObjectInfo
+//
+// If the object has no metadata then metadata will be nil
+func GetMetadata(ctx context.Context, o ObjectInfo) (metadata Metadata, err error) {
+	do, ok := o.(Metadataer)
+	if !ok {
+		return nil, nil
+	}
+	return do.Metadata(ctx)
+}