[ML] AIOps: 8.15 Log Rate Analysis improvements.

[walterra]

Follow up to #172981.

### API
- [x] https://github.com/elastic/kibana/pull/181803
- [x] https://github.com/elastic/kibana/pull/181129
- [x] https://github.com/elastic/kibana/pull/183649
- [x] https://github.com/elastic/kibana/pull/186176

### UI
- [x] Show both a baseline and deviation with doc count in the results table. It might be worth then not showing the raw doc count of the selection, but instead median doc count per histogram bucket to make the numbers comparable. Investigate if we can offer options to show both the raw full number of the selections and/or the median doc count per bucket. https://github.com/elastic/kibana/pull/186342
- [x] https://github.com/elastic/kibana/pull/182793
- [x] https://github.com/elastic/kibana/pull/182690
- [x] https://github.com/elastic/kibana/pull/180969

### Text field pattern support
- [x] Parallelize `fetchCategoryCounts` calls over field names in `fetch_significant_categories.ts` https://github.com/elastic/kibana/pull/183649
- [x] https://github.com/elastic/kibana/pull/186520

### Observability AI Assistant Context
- [x] https://github.com/elastic/kibana/pull/186509

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[walterra]

Closing, created a v8.16.0 follow up #187684 and a general backlog issue #187683.