Avoid Service Usage Consumer role requirement

[schweikert]

gcpdiag sets billing project id for API calls using the X-Goog-User-Project header, either to what was passed as --billing-project, or by default to the project that is being inspected. The reason for this is that otherwise the Oauth client project would be used for billing/quota enforcement.

Unfortunately, this creates a requirement to have the following permission in the inspected project: serviceusage.services.use. That permission is included in the following roles:

• Owner
• Editor
• Service Usage Consumer

Note that Viewer is not enough, so we need to tell users that Viewer+Service Usage Consumer is required.