Add module_depends_on resource to private_service_access

[hronix]

trafficstars

• private_service_access module depends on Service Networking API and vpc_network, so it would be very helpful to have module_depends_on in private_service_access.
• Also description for private_service_access should be updated, it useful not only for MySQL but also for Postgres databases.

[morgante]

Thanks for the suggestion, these both sound like reasonable additions/fixes.