Allow filtering of RDS instances by specifying instance_identifiers in config

### RDS Instance Filtering
When an IAM policy for rds:DescribeDBInstances has Resource restrictions, the rds-describe-db-instances call will fail when an allowed DBInstanceIdentifier is not specified.

If there are no configured instance_identifiers, all available instances will be monitored. This is the default behavior.

```
...
agents:
  rds:
    enabled: true
    instance_identifiers:
      - db1
      - db2
```