Refactor search isolate control + minimal test.

[isoos]

• Part of Create better integration test(s) for search isolate + search handler + search client. #8891. Started using it in fake_server, seems to work, but tests are very flaky with the current code, so submitting only the stable parts and will continue working on the full end-to-end testing in subsequent PRs.
• Updated the local snapshot loading from an arbitrary URL + created a simple web server for it. While the previous file could have been used too, this reduces the complexity of on-demand updates for search.
• In the search entrypoint, the new runSearchInstanceController will use a Stream for triggering index reloads. This allows better control than the previous scheduling, and also provides an easy feedback through the completer.

[sigurdm]

Don't we need to expect something?

[isoos]

At this point the fact that it runs without issues and the future completes is already a good test. I will add search queries and actually changing index at a follow-up pr, when the fake-service is also fixed.

[sigurdm]

Where do you call this with an https url? Or is it preparing for something?

[isoos]

More like completeness. Though, we could make the bucket content available through such URL, which would be https... and that would remove the special case here. While the intention was not to prepare for something, it can be :)