feat(graph): Support dynamic schema

This is to support dynamic schema introduced in https://cloud.google.com/spanner/docs/graph/manage-schemaless-data

```
DYNAMIC LABEL (<label_expr>)
DYNAMIC PROPERTIES (<property_expr>)
```

allows we add labels / properties that are dynamically stored as values (as opposed
to schema objects).

This is useful to manage graph with schema that dynamically evolves
(without making schema changes).

Author: mtyin