Add buildpack_key to cc staging response

Collect the key of the admin buildpack used for staging so it can be sent back to the CC via the staging response.  The goal is to use this information[1] in app usage events once the resolution to story #69196650 is implemented.

[1] https://github.com/cloudfoundry/cloud_controller_ng/blob/4b9fca3aa9782b61515c7993475b21d1e9650ade/lib/cloud_controller/app_stager_task.rb#L27