feat(spanner): add directed reads feature

spanner/client.go

@@ -477,6 +477,8 @@ func (c *Client) BatchReadOnlyTransaction(ctx context.Context, tb TimestampBound
 	t.txReadOnly.qo = c.qo
 	t.txReadOnly.ro = c.ro
 	t.txReadOnly.disableRouteToLeader = true
+	t.txReadOnly.qo.DirectedReadOptions = c.dro
+	t.txReadOnly.ro.DirectedReadOptions = c.dro
 	t.ct = c.ct
 	return t, nil
 }
@@ -508,6 +510,8 @@ func (c *Client) BatchReadOnlyTransactionFromID(tid BatchReadOnlyTransactionID)
 	t.txReadOnly.qo = c.qo
 	t.txReadOnly.ro = c.ro
 	t.txReadOnly.disableRouteToLeader = true
+	t.txReadOnly.qo.DirectedReadOptions = c.dro
+	t.txReadOnly.ro.DirectedReadOptions = c.dro
 	t.ct = c.ct
 	return t
 }