[SEARCH] Refactor fetchSoon and callClient to isolate msearch

[lizozom]

Part of #61738

This is the current search logic flow chart:

To simplify implementation and segregate legacy code, we'd need to refactor the flow to become"

The key changes here are

• Check whether batchSearches is enabled from within SearchSource (this code is to stick around until v8)
• Delete defaultSearchStrategy altogether
• Move fetch, callClient and msearch to a LEGACY folder.
• If batchSearches is disabled, call data's search service directly from SearchSource, otherwise call LEGACY logic

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)