Change credentials to sensitive

[chrisst]

fixes hashicorp/terraform-provider-google#5599

Release Note Template for Downstream PRs (will be copied)

storage: Marked the credentials field in `google_storage_object_signed_url` as sensitive so it doesn't expose private credentials.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 1 file changed, 3 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 1 file changed, 3 insertions(+), 2 deletions(-))