[GCP] Automated credentials creation flow for agentless

[orouz]

in the manual deployment of GCP integration the user is required to provide credentials.

in https://github.com/elastic/kibana/issues/175977 there is an example of how to deploy access_key_id and secret_access_key so the user can copy+paste them easily

this task is for doing the same but for GCP which uses cerdentials json