[Feature request] Network record and replay component

[colintheshots]

On the Fenix team, we would be interested in writing resilient integration tests which are not dependent upon a consistent network experience. Recording and replaying HTTP responses would allow us to record a particular endpoint response and update the test very quickly when it changes. This would let us write fast, highly-performant tests without basing them upon brittle mocks.

In the Android world, Airbnb's OkReplay is one example of an implementation of this idea. https://github.com/airbnb/okreplay

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