Introduce "Failure" response transforms run mode

[alnikola]

Currently, response transforms can be run either on a successful response ("Success" mode) or always ("Always" mode) regardless of the response status. It's suggested to introduce "Failure" run mode for transforms that should be run only on a unsuccessful response or on a failure to get any response. See #1056 for more context.

[Tratcher]

Marking as Breaking-Change because many APIs currently represent Success/Always as a bool that would probobly need to be changed to an enum.
https://github.com/microsoft/reverse-proxy/blob/220b4c890f2905742fc47b16d588dc882f71dfed/src/ReverseProxy/Transforms/ResponseHeaderValueTransform.cs#L15

Would Failure be the opposite of Success? Success currently means any response with a status code less than 4xx.
https://github.com/microsoft/reverse-proxy/blob/220b4c890f2905742fc47b16d588dc882f71dfed/src/ReverseProxy/Transforms/ResponseTransform.cs#L65-L69

[amweiss]

I believe a related use case here we covered by using an exception handler to look for cancelled requests and return a status code 499 to closer mimic nginx for our internal tracking instead of 504 as it normally does.

[adityamandaleeka]

Triage: tentatively moving this to 1.0 since it's a breaking change.