E2Ev2: MitM configurable delay functions

[RXminuS]

Add a configurable delay function to the Man-in-the-Middle proxy middleware. This allows for controlling network timings for proxied requests in test functions.

Also introduce a default floor delay of 10ms for all requests so that recorded / mocked requests avoid flake. This used to happen when requests as they were recorded had reasonable timings whereas replayed requests had their timings set to 0.

Finally we now also keep the original timings in our Polly recordings and instead prepare for more configurable playback speed. This is in anticipation of upcoming "report an issue" features that would require playback at the original speed.

Test plan

CI