[adapters] Use an adapter for serializing entities to a bare python structure
which may be used for e.g. JSON export or other and allow to have different
serializations for each entity types.
Closes #6378035
[dataimport] Fallback from COPY to executemany (closes #9135780)
If we can't build a suitable COPY command (e.g. because data has
unsupported type), use executemany instead. Regression introduced in
3.20.0.