Setup a "plan" presubmit

[mattmoor]

This needs two separate PRS.

1. We should create a NEW identity in iac/bootstrap that's a project viewer and can be federated by pull requests presubmit (e.g. ref:refs/heads/main -> :pull_request). -- I may need to apply this once it lands, or add folks to ACLs.

2. We should add a presubmit like mono that posts the plan to the PR body.

cc @cpanato

[mattmoor]

This failed