feat: Pluggable SQL dialect framework with ClickHouse dialect

Adds a pluggable SQL dialect framework to the OpenSearch SQL plugin's
Calcite-based query engine, with ClickHouse as the first dialect.

Framework:
- DialectPlugin interface, DialectRegistry, QueryPreprocessor in api module
- Dialect routing via ?dialect= query parameter on /_plugins/_sql
- Integration with existing Calcite query pipeline

ClickHouse dialect:
- Query preprocessor strips FORMAT, SETTINGS, FINAL clauses
- ClickHouseOperatorTable translates 25+ ClickHouse functions to Calcite
  equivalents (time-bucketing, type-conversion, aggregates, conditionals)
- OpenSearchClickHouseSqlDialect for RelNode-to-SQL unparsing
- Error handling with position info, function/type identification, safe 500s

Testing:
- 20 property-based tests (jqwik, 100 iterations each)
- Unit tests for registry, routing, error handling
- Integration tests (ClickHouseDialectIT) for end-to-end pipeline
- 43 ClickBench SQL compatibility queries

Signed-off-by: anirudha <anirudha@nyu.edu>
Signed-off-by: Anirudha Jadhav <jadhanir@amazon.com>

Author: anirudha

.kiro/specs/clickhouse-sql-dialect/.config.kiro

@@ -0,0 +1 @@
+{"generationMode": "requirements-first"}