ES|QL LOOKUP JOIN across clusters without replicating lookup indices

## Value Proposition

ES|QL LOOKUP JOIN enriches query results with reference data from a lookup index. In cross-cluster search (CCS) and cross-project search (CPS) scenarios, the current implementation requires the lookup index to exist in every remote cluster — meaning administrators must replicate it via CCR or manual copies. This creates operational burden, and for sensitive data (e.g., employee records, asset inventories), replication may violate data residency or access control requirements.

## Expected Outcome

Users can execute `LOOKUP JOIN` in cross-cluster and cross-project queries where the lookup index exists only on the coordinating cluster. The query planner routes the join to where the lookup data lives rather than requiring copies on every remote cluster, eliminating the replication prerequisite for enrichment workflows.