add an auto setter for gcloud.project.projectNumber
add an auto setter for gcloud.project.projectNumber

This allows kpt to automatically set a project number if the relevant
setter appears in the config, saving users from having to go look it
up themselves. A future change should automatically set this whenever
gcloud.core.project is set.

GoogleContainerTools/kpt,GoogleContainerTools/kpt,GoogleContainerTools/kpt,GoogleContainerTools/kpt,GoogleContainerTools/kpt