compute: expose `ConfidentialInstanceType` field

[msanft]

Is your feature request related to a problem? Please describe.

Currently, the library does not allow to specify the confidential instance type (within the confidential instance config block), while Terraform provider and REST API do.

Describe the solution you'd like

Expose the field in the Go API.

Additional context

cc @deeglaze, who implemented this in the Terraform provider

[msanft]

Perhaps this needs to be adressed in the API spec before being incorporated here? The upstream proto definition doesn't expose the field as well.

[deeglaze]

Unfortunately the googleapis proto definition update from May that many API implementations depend on has been blocked to batch up multiple backward-incompatible changes. It's taking much longer than expected. googleapis/googleapis#903

[codyoss]

These changes should now be rolled out.