RDSInstance doesn't set username in `writeConnectionSecretToRef` if you are importing an existing resource

[trystanleftwich]

What happened?

If I import an existing RDSInstance that has been orphaned, the username in the writeConnectionSecret is never set because its only ever set in the create workflow.

How can we reproduce it?

Create an RDSInstance using

deletionPolicy: Orphan
writeConnectionSecretToRef:
  name: foobar
  namespace: foobar1
forProvider:
...
   masterUsername: foo
   masterPasswordSecretRef:
     key: pass
     name: secret
     namespace: secret
...    
connectionDetails:
    - fromConnectionSecretKey: endpoint
    - fromConnectionSecretKey: port
    - fromConnectionSecretKey: username
    - fromConnectionSecretKey: password

Delete the Secret/manifests

What environment did it happen in?

Crossplane version:
V0.46.0