Add remote GCP consumption via pub/sub

[philhagen]

install the LS Google pub/sub input plugin and provide instructions on getting data from GCP.

"Option 2" here: https://cloud.google.com/solutions/exporting-stackdriver-logging-elasticsearch#configure_logstash

Requires plugin addition: https://www.elastic.co/guide/en/logstash/7.11/plugins-inputs-google_pubsub.html

Configuration files will need to include a stubbed-out sample for a project. Documentation will need to include how to customize a configuration for an individual subscription.

cc @joshlemon for progress tracking and awareness