Rework GCP SQL Instance Access

Reworked how Opal handles access to GCP SQL instances. We now better match how the GCP console behaves:

• Roles are no longer inherited into SQL instance resources.
• The SQL instance user list now purely represents the list of users who have been added to the instance, with no role assignment.
• Adding a user to an SQL instance in Opal also adds the Cloud SQL Instance User role to that user in that instance's parent project.
• Removing a user from an SQL instance in Opal does not remove the Cloud SQL Instance User role from that user.