Set public_key to computed

[c2thorn]

Fixes hashicorp/terraform-provider-google#5552
Fixes hashicorp/terraform-provider-google#5219

google_kms_crypto_key_version: public_key was not set to computed, so terraform did not wait for the datasource to resolve before complaining that the list was empty. The original documentation implies that this field was meant to be implemented as computed.

Release Note Template for Downstream PRs (will be copied)

kms: Fixed issue where `google_kms_crypto_key_version` datasource would throw an Invalid Index error on plan

[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, 1 insertion(+), 1 deletion(-))
Terraform Beta: Diff ( 1 file changed, 1 insertion(+), 1 deletion(-))