[Logs UI] Improve data structures and data access in the log rate feature

[Kerry350]

Summary

In #47558 the log rate feature was refactored to account for new designs and functionality. The log rate results API endpoint currently returns one data structure, and this is transformed into various derivations needed for the UI by functions that exist in the data_formatters file (the complexity is kept to this one location).

This ticket should amend things so that the results hook returns the current data structure, as well as a new data structure that groups the results by partition first and histogram second. With the two data structures provided via the top level we should then be able to dramatically simplify the heavy lifting of the data_formatter functions. We will also increase performance.

This ticket should also simplify the handling of the fake unknown partition (this represents the '' empty string partition). The handling of this scenario and === '' comparisons should either be hoisted to a higher level, or a normalisation function introduced.

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)