Do not assume private key value exists after destroying tfstate

### What happened?

As reported in https://github.com/upbound/provider-gcp/issues/307 and fixed in https://github.com/upbound/provider-gcp/pull/314, upjet can [inject sensitive attributes back to the Terraform state](https://github.com/upbound/upjet/blob/aaafdd4057c5b38feb5b86e4e3d14d646113bfe5/pkg/terraform/files.go#L73), if they are published in a connection details secret referenced by the `spec.writeConnectionSecretToRef` field of a Terraformed resource. However, Terraformed resources do also support publishing connection details to a bunch of external secret stores. We need to consider bringing support for reading back such sensitive Terraform attributes from these external secret stores and injecting them into the Terraform state.

### How can we reproduce it?

Follow the example in https://github.com/upbound/provider-gcp/issues/307