proposal: make the trigger event payload accessible to pipeline workflow runs

In spindle pipelines, you specify triggers like push or pull_request: https://docs.tangled.org/spindles#trigger.

In such a pipeline run, you don't have access to any context about the trigger event.

For example, you can't configure both push and pull_request events and then have conditional logic only for the push event in a step command. As another example, you can't access the commit message for the ref that triggered the push.

A handful of these happen to be available in the environment.

For comparison, GitHub Actions trigger events are unified with their webhooks, so the full webhook payload is available in triggered workflow (example).

Could Tangled do something similar? https://docs.tangled.org/webhooks#webhook-payload