sql: add support for (N) modifier for datatype bpchar

The following throws an error with regards to `bpchar`.

```
create table t (b bpchar(1));
```

but it will work if I remove the N modifier

```
create table t (b bpchar);
```

[slack](https://cockroachlabs.slack.com/archives/C0168LW5THS/p1701796015283839)

Jira issue: CRDB-34207

Epic CRDB-60813

Reproduce the failure with `create table t (b bpchar(1));` and compare it with the working `bpchar` form. Trace how the SQL parser and datatype handling process the `bpchar` modifier, then verify that the parameterized form succeeds without regressing the unparameterized form.

Written by the indexing model from the issue text.