chore: commit logging agent script to repo

[busunkim96]

Fixes b/210461441.

An unvendored shell script (not in your repo) is being executed using curl | bash. All executed source should be commited to your source repository.

This commits what appears to be the latest iteration of the logging agent to the repository.

I visited https://cloud.google.com/logging/docs/agent/logging/installation first, which has https://dl.google.com/cloudagents/add-logging-agent-repo.sh. However, it has this note at the top, so I switched to the Ops Agent:

Key Point: While this agent is still supported, we recommend that you use the Ops Agent for new workloads and eventually transition your existing VMs to use the new agent. The Ops Agent is the eventual replacement for the existing Google Cloud's operations suite agents, and it offers a simple user experience with metrics and logging combined into a single agent.

[engelke]

LGTM