[ML] AIOps Log Rate Analysis: Streaming improvements.

[walterra]

Follow up to #138972.

• [x] The issue #66417 was about fixing support for streaming with Kibana's own http.fetch, now that it's done, we should refactor and try to make use of it instead of the plain fetch. https://github.com/elastic/kibana/pull/162335
• [x] We add the flushFix payload to every message of the ndjson stream. We can improve this by adding the payload periodically only if in a given lookback period we didn't send 4KB of real payload yet https://github.com/elastic/kibana/pull/165069
• [ ] The PR #140506 added a workaround that adds a 4KB dummy payload to each streaming update to properly trigger flushing in certain proxy environments. We need to remove the workaround once these proxies support flushing with smaller payloads.

[elasticmachine]

Pinging @elastic/ml-ui (:ml)