Add AWS RDS Read Replica support for Database Load Balancing

For users who want to setup Database Load Balancing on RDS they will need to provision additional Read Replica database instances separately from the main RDS server but with them configured to replicate them. Note this is different from the standby instance RDS creates as part of it's Multi-AZ feature.

GET currently doesn't have the ability to do this currently as it's separate from the read replica we configure for Geo. Task is to add this.