Add admin username to yml

aws-postgresql.yml

@@ -330,6 +330,11 @@ provision:
       If you do, the encryption is done using an AWS KMS customer managed key.
       This property if provided, will set the customer managed key to use for encrypting the Cloudwatch log group
       created for the RDS PostgreSQL and upgrade logs.
+  - field_name: admin_username
+    type: string
+    details: The username to use for the admin user of the database. When not specified a random username will be generated. This property should only be used when migrating data.
+    default: ""
+    prohibit_update: true
   computed_inputs:
   - name: labels
     default: ${json.marshal(request.default_labels)}