ES|QL Data Federation: Full-text search and relevance ranking on external data

## Value Proposition

At launch, [Data Federation](https://github.com/elastic/roadmap/issues/307) datasets support basic MATCH filtering (no BM25 scoring), and KQL, KNN, and MATCH_PHRASE are not available because external data is not backed by Lucene indices. This limits Data Federation to structured analytics — teams cannot bring Elasticsearch's search capabilities to bear on their external data.

## Expected Outcome

Bring Elasticsearch's full-text search capabilities to external datasets — including relevance-ranked MATCH, phrase matching, and vector search (KNN). This extends Data Federation from a pure analytics tool into a search platform that spans both indexed and external data.