plpgsql: add execution support for OPEN statements

This patch adds support for executing PLpgSQL OPEN statements, which
open a SQL cursor in the current transaction. The name of the cursor
is supplied through a PLpgSQL variable. Since the `REFCURSOR` type
hasn't been implemented yet, this patch uses `STRING` in the
meantime.

Limitations that will be lifted in future PRs:
1. Unnamed cursor declarations are not supported. If a cursor is opened
   with no name supplied, a name should be automatically generated.
2. Bound cursors are not yet supported. It should be possible to declare
   a cursor in the `DECLARE` block with the query already defined, at
   which point it can be opened with `OPEN <cursor>;`.
3. A cursor cannot be opened in a routine with an exception block. This
   is because correct handling of this case is waiting on separate work
   to implement rollback of changes to database state on exceptions.

Informs cockroachdb#109709

Release note (sql change): Added initial support for executing the
PLpgSQL `OPEN` statement, which allows a PLpgSQL routine to create a
cursor. Currently, opening bound or unnamed cursors is not supported.
In addition, `OPEN` statements cannot be used in a routine with an
exception block.

pkg/sql/conn_executor.go

@@ -1222,6 +1222,11 @@ func (ex *connExecutor) close(ctx context.Context, closeType closeType) {
 		ctx, &ex.extraTxnState.prepStmtsNamespaceMemAcc,
 	)
 
+	// Close all cursors.
+	if err := ex.extraTxnState.sqlCursors.closeAll(false /* errorOnWithHold */); err != nil {
+		log.Warningf(ctx, "error closing cursors: %v", err)
+	}
+
 	if closeType == normalClose {
 		// We'll cleanup the SQL txn by creating a non-retriable (commit:true) event.
 		// This event is guaranteed to be accepted in every state.
@@ -1278,9 +1283,6 @@ func (ex *connExecutor) close(ctx context.Context, closeType closeType) {
 			ctx, &ex.extraTxnState.prepStmtsNamespaceMemAcc,
 		)
 		ex.extraTxnState.prepStmtsNamespaceMemAcc.Close(ctx)
-		if err := ex.extraTxnState.sqlCursors.closeAll(false /* errorOnWithHold */); err != nil {
-			log.Warningf(ctx, "error closing cursors: %v", err)
-		}
 	}
 
 	if ex.sessionTracing.Enabled() {
@@ -3865,6 +3867,10 @@ func (ex *connExecutor) txnStateTransitionsApplyWrapper(
 		ex.extraTxnState.prepStmtsNamespaceAtTxnRewindPos.closeAllPortals(
 			ex.Ctx(), &ex.extraTxnState.prepStmtsNamespaceMemAcc,
 		)
+		// Close all cursors.
+		if err := ex.extraTxnState.sqlCursors.closeAll(false /* errorOnWithHold */); err != nil {
+			log.Warningf(ex.Ctx(), "error closing cursors: %v", err)
+		}
 		ex.resetPlanner(ex.Ctx(), &ex.planner, nil, ex.server.cfg.Clock.PhysicalTime())
 	case txnRestart:
 		// In addition to resetting the extraTxnState, the restart event may