[Logs UI] Create primary Logs search strategy and server side foundations

[jasonrhodes]

ℹ️ Full R&D for this work can be found here, and should be read beforehand: #76677 (comment)

Summary

In order to support async search in the Logs UI we want to implement custom search strategies that cover our potentially long-running queries. This should create basis for that effort such that replacements for the various query-specific routes can be rolled out incrementally.

Acceptance criteria

• A discoverable location on the server side holds the search strategy code.
• A search strategy for fetching a single log item in the detail fly-out is registered.
• The search strategy is consumed on the client side.

[elasticmachine]

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

[weltenwort]

Since the discussion in #76677 has shifted towards using multiple single-purpose search strategies for the sake of simplicity, I took the liberty of amending the description above.