Clarification on GetObjects FOREIGN KEY

### What would you like help with?

Is the intent of the `FOREIGN KEY` constraints returned by `AdbcConnectionGetObjects` that they include relational constraints in both directions? That is, for a `CUSTOMER` table defined as in [TPC-H](https://docs.snowflake.com/en/user-guide/sample-data-tpch), should it return both `"ORDERS"."CUSTKEY"` (which references `"CUSTOMER"."CUSTKEY"`) and `"NATION"."NATIONKEY"` (which is referenced by `"CUSTOMER"."NATIONKEY"`)?

If so, the documentation could probably be improved by indicating this explicitly.

If not, if it only returns references in one direction then this is a feature gap with ODBC and JDBC.