sql: support CREATE/DROP DATABASE in stored procedures

**Is your feature request related to a problem? Please describe.**

`CREATE DATABASE` and `DROP DATABASE` are not currently allowed inside PL/pgSQL stored procedure bodies. This is a continuation of the DDL support we have been adding to stored procedures — [#168464](https://github.com/cockroachdb/cockroach/issues/168464) added `CREATE TABLE` / `DROP TABLE`, [#169427](https://github.com/cockroachdb/cockroach/issues/169427) tracks `TRUNCATE TABLE`, and [#172760](https://github.com/cockroachdb/cockroach/issues/172760) tracks `CREATE TABLE AS` — but database-level DDL is not yet supported.

**Describe the solution you'd like**

Allow `CREATE DATABASE` and `DROP DATABASE` inside stored procedure bodies, following the same pattern established for the other DDL statements:

- Add the `CREATE DATABASE` and `DROP DATABASE` AST nodes to the optbuilder allowlist for stored procedures.
- Gate the feature behind the appropriate cluster version.
- Limit the feature to stored procedures (not functions or DO blocks), consistent with the pattern established in [#168464](https://github.com/cockroachdb/cockroach/issues/168464).

**Describe alternatives you've considered**

- Issuing the `CREATE DATABASE` / `DROP DATABASE` statements outside the procedure — works today but defeats the purpose of encapsulating provisioning/teardown logic in a procedure.

**Additional context**

- Part of the ongoing DDL-in-stored-procedures effort (same line of work as [#169427](https://github.com/cockroachdb/cockroach/issues/169427) and [#172760](https://github.com/cockroachdb/cockroach/issues/172760)).
- As noted in [#168464](https://github.com/cockroachdb/cockroach/issues/168464), at compile time all statement references are resolved against the current catalog. This is a known divergence from PostgreSQL's late-binding model and will be addressed separately.

Jira issue: CRDB-66489

Epic CRDB-65937

Read the stored-procedure DDL support from #168464, then start at the optbuilder allowlist for the CREATE DATABASE and DROP DATABASE AST nodes. Check the appropriate cluster-version gate and verify that the statements are accepted in stored procedures but rejected in functions and DO blocks.

Written by the indexing model from the issue text.