Replace calls to external API in e2e tests with something hosted locally on CI

### Description

We're currently calling `GET https://jsonplaceholder.typicode.com/todos/1` and others during e2e tests.
We've just seen a flake where this caused e2e tests to fail.
We should host something on CI for testing locally to not have flaky tests. Similar to the mock Sentry server.

For samples I'd recommend keeping what we currently have and only override for e2e test execution so customers who want to try the sample don't have to spin up another service.