I2I: Cache network requests dispatched from tests

## Summary

Network requests dispatched from tests both slow down test suites and cause instability. By routing these requests through a proxy server which caches the responses we can speed up test runs and reduce flakes.

## Motivation
AMP developers spend a lot of time waiting for tests to run and are forced to repeat this process when flakes occur.
[amphtml Pipelines](https://app.circleci.com/pipelines/github/ampproject/amphtml)
[amphtml Jobs insights](https://app.circleci.com/insights/github/ampproject/amphtml/workflows/CircleCI/jobs?reporting-window=last-90-days&branch=main)

Other options:
* Prevent new tests which use network requests from being added and then gradually removing the requests that exist.
* Update the hosts file to broadly redirect requests and stub them.

/cc @ampproject/wg-approvers

[Design Doc](https://docs.google.com/document/d/1B9fr3Xcs6hDuyhhLmZ-uiHuqN2lZk5n0X5Lp8KRZNAU)

#26754