Add configurable list of users to NOT revoke login

Sometimes when running "revoke-logins", some users needs to be excluded. From this.

It would be nice if this were baked into pgbelt somehow, an ignore list of users when revoking logins.

Start at the pgbelt `revoke-logins` command and trace how users are selected for revocation. Define how a configurable ignore list should be supplied, then verify that listed users are preserved while other users are still revoked. The issue does not name specific files or tests, so locating the command and existing coverage is the first step.

Written by the indexing model from the issue text.