roachtest: run pgbouncer test suite in nightlies

[rafiss]

Try running the tests here: https://github.com/pgbouncer/pgbouncer/tree/master/test

First run locally, then convert into a roachtest script.

Jira issue: CRDB-6433

Epic CRDB-60954

[RichardJCai]

This is probably useful. https://github.com/RichardJCai/crdb-pgbouncer

Also as noted from the meeting, if we want to test multiregion, we want one pgbouncer per region. But that might be a little overkill?

[vy-ton]

@rafiss Once nightlies have been added, are you confident in full support for PgBouncer?