Add option to bgworkers to allow the bypass of role login check

Add option to bgworkers to allow the bypass of role login check

This adds a new option called BGWORKER_BYPASS_ROLELOGINCHECK to the
flags available to BackgroundWorkerInitializeConnection() and
BackgroundWorkerInitializeConnectionByOid().

This gives the possibility to bgworkers to bypass the role login check,
making possible the use of a role that has no login rights while not
being a superuser.  PostgresInit() gains a new flag called
INIT_PG_OVERRIDE_ROLE_LOGIN, taking advantage of the refactoring done in
4800a5dfb4c4.

Regression tests are added to worker_spi to check the behavior of this
new option with bgworkers.

Author: Bertrand Drouvot
Reviewed-by: Nathan Bossart, Michael Paquier, Bharath Rupireddy
Discussion: https://postgr.es/m/bcc36259-7850-4882-97ef-d6b905d2fc51@gmail.com