Add External Secret Stores support in User CRD.

[kaiquerass]

What problem are you facing?

When creating a User for a database, I would like to save the credentials to an External Store, like vault, for example. This is already possible using crossplane (reference), but the User CRD is not prepared to receive this specific configuration.

How could GCP Provider help solve your problem?

Add the ability to use external secret stores when creating a User. Accordingly the docs/example, it's only possible to use k8s secret as the output for the connection details.