Support IDENTITY column creation using existing sequence

In postgres, you can name the sequence which will be created or use an existing schema when using `IDENTITY` syntax with the `SEQUENCE NAME` sequence option. Cockroach does not currently support this option.

For example:

```
CREATE SEQUENCE foo;
CREATE TABLE t (i INT GENERATED BY DEFAULT AS IDENTITY (SEQUENCE NAME foo));
```

It would be great to support this syntax in CockroachDB

#### Additional Context

You can get the same behavior in cockroach and postgres with this syntax:
```
CREATE TABLE t (i INT DEFAULT (nextval('foo')));
```

What cockroach doesn't support is controlling the name of a newly generated sequence.

Jira issue: CRDB-24921