Unify PPL execution in Spark via unified query API

[dai-chen]

Description

This PR introduces the unified-query-spark-integration module that bridges OpenSearch's Calcite-based PPL engine with Apache Spark. This enables consistent PPL query behavior across OpenSearch and Spark by leveraging the unified query API for PPL transpilation. See unified-query-spark-integration/README.md for the architecture overview and usage details.

Key pieces included:

• Custom Spark PPL parser using unified query API: detects PPL input, invokes the unified query planner/transpiler, and forwards the resulting Spark SQL for execution.
• Spark ↔ Calcite catalog schema bridge: maps Spark’s catalog hierarchy (catalogs, databases, tables) into Calcite schema/table abstractions so the unified query planner can resolve relations against Spark metadata.

Note: This module depends on unified-query-api:2.19.4.0-SNAPSHOT from the OpenSearch SQL project. This will be pinned to a released version once the unified query API is stable.

Related Issues

• [FEATURE] Spark catalog integration with PPL-Calcite library #1284
• [FEATURE] Add Spark PPL entry point to route query to PPL-Calcite #1285

Check List

• Updated documentation (docs/ppl-lang/README.md)
• Implemented unit tests
• Implemented tests for combination with other commands
• New added source code should include a copyright header
• Commits are signed per the DCO using --signoff
• Add backport 0.x label if it is a stable change which won't break existing feature

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