workload,cli: Make the UX match that of other client commands

[jseldess]

The new cockroach workload command doesn't match the UX of other client commands in at least the following way:

• Doesn't offer connection flags: --url or --host/--port/--database/--insecure/--certs-dir. See https://www.cockroachlabs.com/docs/dev/connection-parameters.html.

Jira issue: CRDB-6353

[knz]

@jseldess this is because it's not really part of the CockroachDB project, it is functionality imported from a project maintained separately. We do not have a good way to share option processing between them currently. This should probably be addressed towards 2.2.

In the meantime it should really be documented separately from the other CLI commands - it does not share any functionality in common and that distinction should be called out in docs.

[knz]

@awoods187 @vy-ton this issue goes hand-in-hand with #51799