cli: support \watch meta command with the optional iterations counter

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

PG now has support for `\watch` meta command https://www.postgresql.org/message-id/flat/CAAhFRxiZ2-n_L1ErMm9AZjgmUK%3DqS6VHb%2B0SaMn8sqqbhF7How%40mail.gmail.com

Cockroach has support for `-watch` flag but it is assumed the user is not logged into the command line. Would be good to support an inline watch command.

**Describe the solution you'd like**

I should be able to issue `> select now() \watch 2;` and have the `select now()` execute twice.

**Describe alternatives you've considered**

`cockroach sql -watch ...`

**Additional context**
Add any other context or screenshots about the feature request here.

Jira issue: CRDB-24765

Start by tracing the CockroachDB SQL CLI handling for meta commands and the existing -watch flag. Confirm how an inline \watch command with an optional iteration count should execute the preceding select now() statement, and verify that the requested number of executions occurs.

Written by the indexing model from the issue text.