Plan pipeline config

Resolves the steps a build would run for a `.oore.yaml` or the pipeline's form settings, with the same validation as `oore pipeline validate`. Environment values are never returned and secret references stay as written.

curl -X POST "https://example.com/v1/pipelines/plan" \  -H "Content-Type: application/json" \  -d '{}'

{  "available_platforms": [    "android"  ],  "errors": [    "string"  ],  "notices": [    "string"  ],  "plan": null,  "valid": true,  "version": 0,  "workflows": [    "string"  ]}