Mirror chdb-core symbols: DataStore output_format + UDF (create_function, NullHandling, ...)

[wudidapaopao]

chdb-ds's chdb/__init__.py currently lacks two groups of top-level symbols that exist in chdb-core:

1. DataStore output_format (paired with Support output_format="DataStore" in query / session / streaming APIs chdb-core#67 / Fix arrow jemalloc #68):

   • chdb.query(SQL, output_format="DataStore") should return a DataStore

2. UDF module-level API (already in chdb-core, never mirrored in chdb-ds):

   • chdb.create_function
   • chdb.drop_function
   • chdb.NullHandling / chdb.ExceptionHandling
   • chdb.func (from chdb.udf.func)

The UDF group does not exist in chdb-core release 26.3.0; the mirror must use hasattr guards so chdb-ds stays backward-compatible with older chdb-core releases.