feat: Add pre-pipeline-upload hook

[thejcannon]

👋 Howdy!

We would love it if there was a standard hook that existed for "pre- pipeline upload" such that the hook would get a chance to customize the YAML before the agent uploads it.

The use case is that we rely quite heavily on agent_query_rules, and allow folks to upload pipelines with less-than-the-required-amount of rules. The hook would allow us to plug the expected query rules in (with defaults).