prow: Draft Prow proposal and experiment

[sprt]

Main concern: keeping costs low.

[afinn12]

[WIP] Prow Proposal: https://docs.google.com/document/d/17NKrTg2wb3wdK0GHcbD42jv2SURneC93thcfk6ZO1ZI/edit?usp=sharing

[JamesKnee]

Updated Prow Proposal: https://docs.google.com/document/d/17UyHFISMm3XhTnqK5VWlg2GftkLSPbJdtMezvP_CWqc/edit?usp=sharing