sql: internal error returning RECORD of REFCURSORS in UDF

The following snippet results in an internal error:

```sql
CREATE TABLE t (
k INT PRIMARY KEY,
v INT
);

CREATE OR REPLACE FUNCTION p(ref1 REFCURSOR, ref2 REFCURSOR)
RETURNS RECORD
LANGUAGE PLPGSQL
AS $$
BEGIN
OPEN ref1 FOR SELECT k, v FROM t;
OPEN ref2 FOR SELECT k, v FROM t;
RETURN (ref1, ref2);
END
$$;

SELECT * FROM p('mycursor1', 'mycursor2') AS p(c1 string, c2 string);
```

```
ERROR: internal error: invalid datum type given: RECORD, expected STRING
SQLSTATE: XX000
DETAIL: stack trace:
github.com/cockroachdb/cockroach/pkg/sql/rowenc/encoded_datum.go:195: DatumToEncDatum()
github.com/cockroachdb/cockroach/pkg/sql/rowexec/project_set.go:334: toEncDatum()
github.com/cockroachdb/cockroach/pkg/sql/rowexec/project_set.go:241: nextGeneratorValues()
github.com/cockroachdb/cockroach/pkg/sql/rowexec/project_set.go:313: Next()
github.com/cockroachdb/cockroach/pkg/sql/colexec/columnarizer.go:239: Next()
github.com/cockroachdb/cockroach/pkg/sql/colflow/stats.go:118: next()
github.com/cockroachdb/cockroach/pkg/sql/colexecerror/error.go:92: CatchVectorizedRuntimeError()
github.com/cockroachdb/cockroach/pkg/sql/colflow/stats.go:126: Next()
github.com/cockroachdb/cockroach/pkg/sql/colflow/flow_coordinator.go:250: nextAdapter()
github.com/cockroachdb/cockroach/pkg/sql/colexecerror/error.go:92: CatchVectorizedRuntimeError()
github.com/cockroachdb/cockroach/pkg/sql/colflow/flow_coordinator.go:254: next()
github.com/cockroachdb/cockroach/pkg/sql/colflow/flow_coordinator.go:286: Run()
github.com/cockroachdb/cockroach/pkg/sql/colflow/vectorized_flow.go:320: Run()
github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:910: Run()
github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:1957: PlanAndRun()
github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:1672: func3()
github.com/cockroachdb/cockroach/pkg/sql/distsql_running.go:1675: PlanAndRunAll()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:2393: execWithDistSQLEngine()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:1931: dispatchToExecutionEngine()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:1108: execStmtInOpenState()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:146: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:3374: execWithProfiling()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor_exec.go:145: execStmt()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:2225: func1()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:2230: execCmd()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:2147: run()
github.com/cockroachdb/cockroach/pkg/sql/conn_executor.go:953: ServeConn()
github.com/cockroachdb/cockroach/pkg/sql/pgwire/conn.go:247: processCommands()
github.com/cockroachdb/cockroach/pkg/sql/pgwire/server.go:1001: func3()
src/runtime/asm_arm64.s:1197: goexit()
```

cc @a-entin

Jira issue: CRDB-35312