Figure out future of `/retest` app

[phlax]

We made a load of changes to the retest app recently to work with Envoy's new CI framework

This may have broken some users as the logic is now more tied to Envoy's checks and essentially the github/checks action which has related logic

It was my intention to fold the action into that one

I reckon we can support a generic /retest one way/other so i guess its a case of figuring out how/where to maintain the code

[phlax]

cc @mmorel-35

cc @zirain

[zirain]

EG(envoy gateway) use retest app, and EG use github action only.
I'm glad to see this happen, and will help to maintain it as possible as I can.

[phlax]

i spent some time looking at this at the end of last week

essentially Envoys needs are different - it looks for failed checks rather than runs and then uses the checks to restart the runs

im thinking the best way forward is to shift the envoy code to the github/checks app, and then ~revert this to the pre-change version (and remvoe the azp code)

ill update as i have some time

[zirain]

Thanks for your updates, one it's ready, please ping me to give a try.