feat: DataFusion query layer for parquet metrics

[alexanderbianchi]

Adds a DataFusion-based query execution layer on top of the wide-schema parquet metrics pipeline from PR #6237.

• New crate quickwit-datafusion with pluggable QuickwitDataSource trait, DataFusionSessionBuilder, QuickwitSchemaProvider, and distributed worker session setup via datafusion-distributed WorkerService
• MetricsDataSource implements QuickwitDataSource for OSS parquet splits: metastore-backed split discovery, object-store caching, filter pushdown with CAST unwrapping fix, Substrait ReadRel consumption
• DataFusionService gRPC (ExecuteSql + ExecuteSubstrait streaming) wired into quickwit-serve alongside the existing searcher and OTLP services
• Distributed execution: DistributedPhysicalOptimizerRule produces PartitionIsolatorExec tasks (not shuffles) across multiple searcher nodes
• Integration tests covering: pruning, aggregation, time range, GROUP BY, distributed tasks, NULL column fill for missing parquet fields, Substrait named-table queries, rollup from file, partial schema projection
• dev/ local cluster setup with start-cluster, ingest-metrics, query-metrics

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