rcat: preserve metadata when Copy falls back to Rcat

Before this change if we copied files of unknown size, then they lost
their metadata.

This was particularly noticeable using --s3-decompress.

This change adds metadata to Rcat and RcatSized and changes Copy to
pass the metadata in when it calls Rcat for an unknown sized input.

Fixes #6546

vfs/vfscache/downloaders/downloaders_test.go

@@ -85,7 +85,7 @@ func TestDownloaders(t *testing.T) {
 
 	// Write the test file
 	in := io.NopCloser(readers.NewPatternReader(size))
-	src, err := operations.RcatSize(ctx, r.Fremote, remote, in, size, time.Now())
+	src, err := operations.RcatSize(ctx, r.Fremote, remote, in, size, time.Now(), nil)
 	require.NoError(t, err)
 	assert.Equal(t, size, src.Size())