sql: support nested domain types (domain of domain)

## Summary

PostgreSQL supports creating a domain whose base type is another domain. When this happens, constraints from all domains in the chain are enforced. CockroachDB should support this for full PostgreSQL compatibility.

## PostgreSQL Behavior

```sql
CREATE DOMAIN positive_int AS INT CHECK (VALUE > 0);
CREATE DOMAIN small_positive_int AS positive_int CHECK (VALUE < 100);

SELECT (-5)::small_positive_int;
-- ERROR: value for domain positive_int violates check constraint "positive_int_check"

SELECT 200::small_positive_int;
-- ERROR: value for domain small_positive_int violates check constraint "small_positive_int_check"

SELECT 50::small_positive_int;
-- OK
```

Both the parent domain's constraints and the child domain's constraints are enforced.

## Current State

As of #165187, `CREATE DOMAIN d2 AS d1` is blocked with an unimplemented error when `d1` is a domain type. This issue tracks removing that restriction and properly supporting nested domains.

## Implementation Notes

Key considerations:
- Constraint flattening vs. chain-walking at validation time
- NOT NULL and DEFAULT inheritance from parent domains
- Back-reference management across the domain chain
- `DROP TYPE CASCADE` behavior when a parent domain is dropped

Relates to #27796.

Jira issue: CRDB-61352

Epic CRDB-66030