sql: support DDL in read committed transactions

We currently do not allow DDL statements like CREATE TABLE, CREATE SCHEMA, CREATE INDEX, etc. within explicit read committed transactions when `autocommit_before_ddl` is off. Postgres allows these statements within read committed transactions.

A demonstration is something like:

```sql
SET autocommit_before_ddl = off;
SET CLUSTER SETTING sql.txn.read_committed_isolation.enabled = true;
BEGIN TRANSACTION ISOLATION LEVEL READ COMMITTED;
SELECT 1;
CREATE TABLE a (a INT PRIMARY KEY);
ROLLBACK;
```

In CRDB this fails with `explicit transaction involving a schema change needs to be SERIALIZABLE`.

Jira issue: CRDB-33677

Epic CRDB-60942

Start by running the SQL reproduction from the issue with autocommit_before_ddl disabled and READ COMMITTED enabled. Trace where CockroachDB rejects schema changes in this transaction mode, then verify that CREATE TABLE and the other listed DDL statements are accepted and that the demonstrated transaction can roll back as expected.

Written by the indexing model from the issue text.